13.13: Embryological Development - Biology

13.13: Embryological Development - Biology

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Most animal species undergo a separation of tissues into germ layers during embryonic development. These animals are called diploblasts. Animals with three tissue layers are called triploblasts.

Practice Question

Which of the following statements about diploblasts and triploblasts is false?

  1. Animals that display radial symmetry are diploblasts.
  2. Animals that display bilateral symmetry are triploblasts.
  3. The endoderm gives rise to the lining of the digestive tract and the respiratory tract.
  4. The mesoderm gives rise to the central nervous system.

[reveal-answer q=”815922″]Show Answer[/reveal-answer]
[hidden-answer a=”815922″]Statement d is false.[/hidden-answer]

Each of the three germ layers is programmed to give rise to particular body tissues and organs. The endoderm gives rise to the lining of the digestive tract (including the stomach, intestines, liver, and pancreas), as well as to the lining of the trachea, bronchi, and lungs of the respiratory tract, along with a few other structures. The ectoderm develops into the outer epithelial covering of the body surface, the central nervous system, and a few other structures. The mesoderm is the third germ layer; it forms between the endoderm and ectoderm in triploblasts. This germ layer gives rise to all muscle tissues (including the cardiac tissues and muscles of the intestines), connective tissues such as the skeleton and blood cells, and most other visceral organs such as the kidneys and the spleen.

Presence or Absence of a Coelom

Further subdivision of animals with three germ layers (triploblasts) results in the separation of animals that may develop an internal body cavity derived from mesoderm, called a coelom, and those that do not. This epithelial cell-lined coelomic cavity represents a space, usually filled with fluid, which lies between the visceral organs and the body wall. It houses many organs such as the digestive system, kidneys, reproductive organs, and heart, and contains the circulatory system. In some animals, such as mammals, the part of the coelom called the pleural cavity provides space for the lungs to expand during breathing. The evolution of the coelom is associated with many functional advantages. Primarily, the coelom provides cushioning and shock absorption for the major organ systems. Organs housed within the coelom can grow and move freely, which promotes optimal organ development and placement. The coelom also provides space for the diffusion of gases and nutrients, as well as body flexibility, promoting improved animal motility.

Triploblasts that do not develop a coelom are called acoelomates, and their mesoderm region is completely filled with tissue, although they do still have a gut cavity. Examples of acoelomates include animals in the phylum Platyhelminthes, also known as flatworms. Animals with a true coelom are called eucoelomates (or coelomates) (Figure 2). A true coelom arises entirely within the mesoderm germ layer and is lined by an epithelial membrane. This membrane also lines the organs within the coelom, connecting and holding them in position while allowing them some free motion. Annelids, mollusks, arthropods, echinoderms, and chordates are all eucoelomates. A third group of triploblasts has a slightly different coelom derived partly from mesoderm and partly from endoderm, which is found between the two layers. Although still functional, these are considered false coeloms, and those animals are called pseudocoelomates. The phylum Nematoda (roundworms) is an example of a pseudocoelomate. True coelomates can be further characterized based on certain features of their early embryological development.

Embryonic Development of the Mouth

Bilaterally symmetrical, tribloblastic eucoelomates can be further divided into two groups based on differences in their early embryonic development. Protostomes include arthropods, mollusks, and annelids. Deuterostomes include more complex animals such as chordates but also some simple animals such as echinoderms. These two groups are separated based on which opening of the digestive cavity develops first: mouth or anus. The word protostome comes from the Greek word meaning “mouth first,” and deuterostome originates from the word meaning “mouth second” (in this case, the anus develops first). The mouth or anus develops from a structure called the blastopore (Figure 3).

The blastopore is the indentation formed during the initial stages of gastrulation. In later stages, a second opening forms, and these two openings will eventually give rise to the mouth and anus (Figure 3). It has long been believed that the blastopore develops into the mouth of protostomes, with the second opening developing into the anus; the opposite is true for deuterostomes. Recent evidence has challenged this view of the development of the blastopore of protostomes, however, and the theory remains under debate.

Another distinction between protostomes and deuterostomes is the method of coelom formation, beginning from the gastrula stage. The coelom of most protostomes is formed through a process called schizocoely, meaning that during development, a solid mass of the mesoderm splits apart and forms the hollow opening of the coelom. Deuterostomes differ in that their coelom forms through a process called enterocoely. Here, the mesoderm develops as pouches that are pinched off from the endoderm tissue. These pouches eventually fuse to form the mesoderm, which then gives rise to the coelom.

The earliest distinction between protostomes and deuterostomes is the type of cleavage undergone by the zygote. Protostomes undergo spiral cleavage, meaning that the cells of one pole of the embryo are rotated, and thus misaligned, with respect to the cells of the opposite pole. This is due to the oblique angle of the cleavage. Deuterostomes undergo radial cleavage, where the cleavage axes are either parallel or perpendicular to the polar axis, resulting in the alignment of the cells between the two poles.

There is a second distinction between the types of cleavage in protostomes and deuterostomes. In addition to spiral cleavage, protostomes also undergo determinate cleavage. This means that even at this early stage, the developmental fate of each embryonic cell is already determined. A cell does not have the ability to develop into any cell type. In contrast, deuterostomes undergo indeterminate cleavage, in which cells are not yet pre-determined at this early stage to develop into specific cell types. These cells are referred to as undifferentiated cells. This characteristic of deuterostomes is reflected in the existence of familiar embryonic stem cells, which have the ability to develop into any cell type until their fate is programmed at a later developmental stage.

Try It

One of the first steps in the classification of animals is to examine the animal’s body. Studying the body parts tells us not only the roles of the organs in question but also how the species may have evolved. One such structure that is used in classification of animals is the coelom. A coelom is a body cavity that forms during early embryonic development. The coelom allows for compartmentalization of the body parts, so that different organ systems can evolve and nutrient transport is possible. Additionally, because the coelom is a fluid-filled cavity, it protects the organs from shock and compression. Simple animals, such as worms and jellyfish, do not have a coelom. All vertebrates have a coelom that helped them evolve complex organ systems.

Animals that do not have a coelom are called acoelomates. Flatworms and tapeworms are examples of acoelomates. They rely on passive diffusion for nutrient transport across their body. Additionally, the internal organs of acoelomates are not protected from crushing.

Animals that have a true coelom are called eucoelomates; all vertebrates are eucoelomates. The coelom evolves from the mesoderm during embryogenesis. The abdominal cavity contains the stomach, liver, gall bladder, and other digestive organs. Another category of invertebrates animals based on body cavity is pseudocoelomates. These animals have a pseudo-cavity that is not completely lined by mesoderm. Examples include nematode parasites and small worms. These animals are thought to have evolved from coelomates and may have lost their ability to form a coelom through genetic mutations. Thus, this step in early embryogenesis—the formation of the coelom—has had a large evolutionary impact on the various species of the animal kingdom.


Embryology is the branch of biology concerned with the development of new organisms. Embryologists track reproductive cells (gametes) as they progress through fertilization, become a single-celled zygote, then an embryo, all the way to a fully functioning organism. There are many subdivisions of embryology, some scientist focusing on human embryos, while others study animals and plants. Evolutionary biologists often use embryology as a means of comparing species, as the development of an organism can give many clues to its evolutionary history. Still other scientists use embryology as a tool to better understand the system or organism they are dealing with, be it conservation of an endangered species or the reproductive disruption of a pest species. Scientists studying human embryology assist with women’s reproductive health, and understand the broad scope of issues which can lead to developmental defects and malformations.

Embryology Questions for CSIR JRF Life Sciences Examination

Dear Students,
Welcome to Developmental Biology MCQ – 04. This MCQ set consists of Advanced (Post Graduate Level) Developmental Biology / Embryology Multiple Choice Questions with Answer Key. All these questions were taken from the previous year question papers of CSIR JRF NET Life Sciences Examination. These questions can be used for the preparation of Competitive examinations in Biology / Life Sciences such as CSIR JRF NET, ICMR JRF, DBT BET JRF, GATE and other University Ph.D Entrance Examinations. After marking your answers, please click ‘ SUBMIT ‘ button to see your ‘ SCORE ‘ and ‘ CORRECT ANSWERS ‘.

(1). Cancer is often believed to arise from stem cells rather than fully differentiated cells. Following are certain views related to the above statement. Which one of the following is NOT correct?

(a). Stem cells do not divide and therefore require fewer changes to become a cancer cell.
(b). Cancer stem cells can self-renew as well as generate the non-stem cell populations of the tumor.
(c). Teratocarcinomas prove tumors arise from stem cells without further mutations.
(d). Stemness genes can often function as oncogenes.

(2). Given are certain facts which define ‘determination’ of a developing embryo.

(A). Cells have made a commitment to a differentiation program.
(B). A phase where specific biochemical actions occur in embryonic cells.
(C). The cell cannot respond to differentiation signals.
(D). A phase where inductive signals trigger cell differentiation.

Which of the above statements best define determination?

(3). What would happen as a result of a transplantation experiment in a chick embryo where the leg mesenchyme is placed directly beneath the wing apical ectodermal ridge (AER)?

(a). Distal hindlimb structures develop at the end of the limb.
(b). A complete hindlimb will form in the region where the forelimb should be.
(c). The forelimb would form normally.
(d). Neither a forelimb nor a hindlimb would form since the cells are already determined.

(4). If you remove a set of cells from an early embryo, you observe that the adult organism lacks the structure that would have been produced from those cells. Therefore, the organism seems to have undergone.

(a). Autonomous specification.
(b). Conditional specification.
(c). Morphogenic specification.
(d). Syncytial specification.

(5). Dose-dependence of retinoic acid treatment supports the notion that a gradient of retinoic acid can act as a morphogen along the proximo-distal axis in a developing limb. Following are certain facts related to the above notion.

(A). Treatment with high level of retinoic acid causes a proximal blastema to be respecified as a distal blastema and only distal structures are regenerated.
(B). Treatment with high level of retinoic acid causes a distal blastema to be respecified as a proximal blastema and regeneration of a full limb may be initiated.
(C). Treatment with retinoic acid affects only distal blastemas and causes them to form only proximal structures.
(D). Treatment with high level of retinoic acid causes any blastema to form only distal structures.

Which one of the following is correct?

(6). According to the ABC model of floral development in Arabidopsis as shown below,

Several genes/transcription factors e.g. AP1, AP2, AP3, AG etc., are involved. Which one of the following statements is correct?

(a). Apetala 2 (AP2) transcripts expressed during sepal and petal development.
(b). Agamous AG is considered as class A gene.
(c). AP1 expressed during carpel development.
(d). AP3 expressed during sepal development.

(7). Match the following cleavage patterns with the species in which they occur. (CSIR_2017_II)

(a). A – iv, B – iii, C – i, D – ii
(b). A – iii, B – i, C – iv, D – ii
(c). A – ii, B – iii, C – i, D – iv
(d). A – ii, B – iv, C – iii, D – i

(8). Which one of the following statements regarding limb development in mice is true?

(a). The gene encoding Tbx5 is transcribed in the limb fields of the hindlimbs.
(b). The gene encoding Tbx4 is transcribed in the limb fields of the forelimbs.
(c). Genes encoding Islet 1, Tbx4 and Pitx are expressed in the presumptive hindlimb.
(d). Genes encoding Islet 1, Tbx4 and Pitx are expressed in the presumptive forelimb.

(9). C. elegans embryo uses both autonomous and conditional modes of specification. Conditional specification at the 4-cell stage can be seen in the development of the endoderm cell lineage and also in the establishment of dorsal-ventral axis. Following are few statements regarding this:

(A). If the P2 cell is removed at the early 4-cell stage, the EMS cell will divide into two MS cells and no endoderm will be made.
(B). In pop-1 deficient embryos, both EMS daughter cells become E cells.
(C). When the position of ABa and ABp was reversed, their fates get reversed and no normal embryo forms.
(D). In embryos whose mother have mutant glp-1, ABp is transformed into ABa cell.

Species Comparison of Carnegie Stages

This table shows a comparison between different animal embryos and human embryos using the same staging criteria. Note that researchers have also developed embryo staging criteria that is specific to a single species.

  1. Download and print the linked file.
  2. Use the graph as a guide to complete the table.
  3. Colour the stage table to show each developmental week range for each species over the weeks 1 to 8. (The chicken has been given as an example in the table)

Stages of Mammalian Embryonic Development

Although there are some inherent differences between species, the embryos of most vertebrate species involve the same processes during embryogenesis. Most of the notable differences tend to become more apparent during the later stages of development. In mammals, embryogenesis proceeds in the following distinct stages:


Following fertilization, the zygote begins to divide by mitosis in a manner in which there is a lack of growth, and the resulting cluster of cells remains the same size as the initial fertilized cell (shown below). After four rounds of cleavage, the 16-celled cluster is termed the morula. The cells comprising the morula eventually form an outer layer called the trophoblast and an inner cluster of cells, termed the inner cell mass, which will form the embryo. Fluid will then fill the space between the trophoblast and the inner cells, with the two cell formations connecting at one pole, termed the embryonic pole.

Blastula Stage

After seven rounds of cleavage, the cell cluster comprised of 128 cells is known as the blastula. The blastula is characterized by a circular layer of cells termed the blastoderm surrounding an inner cell mass termed the blastocyst (shown below). The fluid-filled cavity residing between the two groups of cells is termed the blastocoel. During this stage, the trophoblast as described above is divided into an outer layer called the syncytiotrophoblast and an inner layer termed the cytotrophoblast. These layers do not form the embryo, but will eventually help form the placenta. The inner cluster of cells, termed the inner cell mass, also begins to undergo organization during this stage. At the center of the inner cell mass is a layer of flat, differentiated cells termed the endoderm. The endoderm forms the yolk sac which will supply the growing embryo with nutrients and a source of blood supply until the formation of the placenta is complete. Between the remaining cells, the amniotic cavity is formed, the bottom of which is composed of prismatic cells called the ectoderm, and forms a structure called the embryonic disk. The embryonic disk then begins to change conformation and forms a pore with the yolk sac. The cells of the ectoderm gradually descend to meet the endoderm. A third layer of cells is also formed and is situated laterally between the endoderm and ectoderm. These layers are termed the germ layers and will eventually form the various tissues of the organism. It is also during this stage that implantation of the embryo into the uterine wall occurs.

Gastrula Stage

Once the three germ layers have been formed and move towards the center of the blastula, the embryo is called a gastrula (shown below). Although the differentiation of the various cell types occurs during the blastula stage, the organization of the cell into three distinct layers is known as gastrulation. Gastrulation typically occurs during the third week of pregnancy, and the process begins with the formation of a thick structure along the midline of the embryonic disk, termed the primitive streak. The primitive streak defines the major axes of the embryo (left, right, cranial, and caudal sides). At the cranial end of the embryonic disk, the primitive streak expands to form a primitive node and begins to extend along the midline to the caudal end and to form a primitive groove. At this point, the outer layer of cells begins to fold inward and detach along the primitive streak via a process termed invagination. The first cells which move inward displace the outer layer of cells and are replaced by a new cell layer termed the definitive endoderm. Inside the embryo, the cells that were internalized join and form the definitive ectoderm. The group of cells residing between the definitive ectoderm and endoderm form the definitive mesoderm.

Organogenesis Stage

Human Embryology and Developmental Biology

Master the concepts you need to know with Human Embryology and Developmental Biology. Dr. Bruce M. Carlson's clear explanations provide an easy-to-follow "road map" through the most up-to-date scientific knowledge, giving you a deeper understanding of the key information you need to know for your courses, exams, and ultimately clinical practice.

Master the concepts you need to know with Human Embryology and Developmental Biology. Dr. Bruce M. Carlson's clear explanations provide an easy-to-follow "road map" through the most up-to-date scientific knowledge, giving you a deeper understanding of the key information you need to know for your courses, exams, and ultimately clinical practice.

Key Features

  • Visualize normal and abnormal development with hundreds of superb clinical photos and embryological drawings.
  • Access the fully searchable text online, view animations, answer self-assessment questions, and much more at
  • Grasp the molecular basis of embryology, including the processes of branching and folding - essential knowledge for determining the root of many abnormalities.
  • Understand the clinical manifestations of developmental abnormalities with clinical vignettes and Clinical Correlations boxes throughout.

Your purchase entitles you to access the web site until the next edition is published, or until the current edition is no longer offered for sale by Elsevier, whichever occurs first. If the next edition is published less than one year after your purchase, you will be entitled to online access for one year from your date of purchase. Elsevier reserves the right to offer a suitable replacement product (such as a downloadable or CD-ROM-based electronic version) should access to the web site be discontinued.

  • Visualize normal and abnormal development with hundreds of superb clinical photos and embryological drawings.
  • Access the fully searchable text online, view animations, answer self-assessment questions, and much more at
  • Grasp the molecular basis of embryology, including the processes of branching and folding - essential knowledge for determining the root of many abnormalities.
  • Understand the clinical manifestations of developmental abnormalities with clinical vignettes and Clinical Correlations boxes throughout.

Your purchase entitles you to access the web site until the next edition is published, or until the current edition is no longer offered for sale by Elsevier, whichever occurs first. If the next edition is published less than one year after your purchase, you will be entitled to online access for one year from your date of purchase. Elsevier reserves the right to offer a suitable replacement product (such as a downloadable or CD-ROM-based electronic version) should access to the web site be discontinued.

Top 16 Stages of Embryology in Plants (With Diagram)

The following points highlight the top sixteen stages of embryology in plants. Some of the stages are: 1. T.S. Young (developing) Anther 2. T.S. Anther Showing Four Mature Pollen Sacs 3. T.S. Mature Anther Showing Dehiscence 4. Pollen Tetrads 5. Pollen Grain 6. Ovule Types 7. L.S. Anatropous Ovule 8. Archesporial Initial 9. Two-celled Stage of Megaspore Mother Cell 10. Linear Tetrad of Megaspores and a few others.

Embryology in Plants: Stage # 1.

T.S. Young (developing) Anther:

1. It is a multicellular, four-cornered structure, sur­rounded by a layer of epidermis.

2. In each corner develops one or more archesporial initials.

3. These initials divide by a periclinal wall into outer primary parietal cell and inner primary sporogenous cell.

4. Primary parietal cell divides periclinally as well as anticlinally and form 3 to 5 concentric layers of cells.

5. Innermost wall layer is called tapetum which is nutritive in function.

6. From the sporogenous tissue develop the pollen grains.

7. Some cells form the procambial strand in the centre of the anther.

Embryology in Plants: Stage # 2.

T.S. Anther Showing Four Mature Pollen Sacs:

1. It is a four-cornered structure containing a pollen sac (Fig. 182).

2. Anther is surrounded by a layer of epidermis throughout.

3. Each pollen sac is surrounded by epidermis, an endothecial layer, one to three middle layers or wall layers and innermost layer of tapetum.

4. In each pollen sac or pollen chamber are present many pollen tetrads which on separation form microspores.

5. A joint in the form of connective is present in the centre.

Embryology in Plants: Stage # 3.

T.S. Mature Anther Showing Dehiscence:

1. It is a four-cornered, four-chambered, multicellular body surrounded by a layer of epidermis.

2. Partition wall between the two pollen sacs is dis­solved (Fig. 183).

3. Many pollen grains or microspores are present in the pollen sacs in the form of fine, powdery or granu­lar mass.

4. Endothecium, middle layers and tapetal layers are present below the epidermis.

5. Along the line of dehiscence of each lobe, thin- walled cells of endothecium form the stomium.

6. A connective is very clear.

Embryology in Plants: Stage # 4.

(A) Isobilateral Tetrad:

All the four spores are formed in one plane because the spindles of first and sec­ond meiotic division remain at right angle to one another (Fig. 184), e.g.,Zea mays.

Out of the two lower spores, only one is visible. Both the upper ones are clear (Fig. 184), e.g.,Magnolia.

In meiosis II upper cell divides to form two cells present side by side and the lower cell forms two cells lying one above the other, e.g., Aristolochia.

(D) Linear Tetrad:

All the four spores are present one above the other in a linear fashion, e.g., Halophila.

(E) Compound Pollen Grain:

Sometimes microspore tetrads adhere to each other (Fig. 184) and form the compound pollen grain, e.g., Typha, Cryptostegia.

Pollen grains of a pollen sac sometimes remain together to form a single mass called pollinium. Each pollinium (Fig. 184) consists of carpusculum, caudicle and pollinia, e.g., Asclepiadaceae.

Embryology in Plants: Stage # 5.

1. It is a unicellular, uninucleate structure (Fig. 185). But pollen grains are always 2- or-3 nucleate, when shed.

2. It is surrounded by a double-layered wall, i. e., outer exine and inner intine.

3. Exine is thick, cutinized, pigmented, sculptured and perforated by germ pores.

4. Intine is thin, colourless, smooth and consists of cellulose.

5. In the cytoplasm are present water, protein, fats, carbohydrates, etc.

Embryology in Plants: Stage # 6.

(Ortho, straight tropous, turned). When micropyle, chalaza and funicle lie in one straight line e.g., Polygonaceae, Urticaceae.

(Ana, backwards tropous, turned). Here, the body of the ovule turns backwards by an angle of 180° and so the micropyle becomes close to the hylum and placenta Sympetalae.

(Hemi, half tropous, turned). Here the body of the ovule is placed transversely or somewhat at right angle to the funicle. Chalaza and micropyle are present here in one straight line (Fig. 186) e.g., Ranunculus.

(Kampylos, curved). Here the body of the ovule is curved in such a way that the chalaza and the micropyle do not lie in the same straight line e.g., Leguminosae.

Here the curvature of ovule is more pronounced and embryo sac becomes horse­shoe shaped (Fig. 186) e.g., Butomaceae.

Here the funicle is very long and the ovule rotates by an angle of 360° in such a fashion that it is completely circled around by the funicle. Micropyle faces upward e.g., Cactaceae.

Embryology in Plants: Stage # 7.

1. It is attached to the placenta with a stalk called funicle.

2. The point of attachment of funicle with the body of the ovule is known as hilum which extends above in the form of a ridge called raphe.

3. Nucellus consists of parenchymatous cells.

4. Nucellus remains covered by one or two coverings called integuments.

5. Integuments remain disconnected at one point form­ing a passage called micropyle.

6. Embryo sac consists of three antipodals, two synergids, one egg cell and one secondary nucleus.

7. Antipodals are located near the chalaza end and the egg cell and synergids towards the micropylar end.

Embryology in Plants: Stage # 8.

1. It is hypodermal in origin.

2. Archesporial initial is bigger than that of its sur­rounding cells.

3. A conspicuous nucleus and dense cytoplasm is present in it.

4. In its later stages, it divides into two cells forming an outer parietal cell which forms the parietal tis­sue and inner megaspore mother cell.

Embryology in Plants: Stage # 9.

Two-celled Stage of Megaspore Mother Cell:

1. Two cells are present one above the other (Fig. 189).

2. These are formed after reduction division and so each cell contains haploid set of chromosomes.

3. Tetrad is formed from these two cells.

Embryology in Plants: Stage # 10.

Linear Tetrad of Megaspores:

1. Four megaspores are arranged in linear fashion.

2. These are haploid in nature.

3. Out of the four megaspores, only one remains func­tional which is near the chalazal end. Remaining three degenerate (Fig. 190).

4. Functional megaspore is the first cell of the female gametophyte and it develops into the embryo sac.

Embryology in Plants: Stage # 11.

Ovule with Binucleate Embryo-Sac:

1. Two nuclei are present in the embryo sac.

2. These two nuclei are formed by the division of the nucleus of the functional megaspore.

3. After some time two nuclei are separated by a large vacuole and they reach at the corners.

Embryology in Plants: Stage # 12.

Ovule with 4-Nucleate Embryo-Sac:

1. Four nuclei are present in the embryo sac (Fig. 192).

2. Out of the four nuclei, two are present near the chalazal end and the remaining two nuclei near the micropylar end.

3. In the centre is present a large central vacuole.

4. Traces of degenerated megaspores are also seen at the micropylar end.

Embryology in Plants: Stage # 13.

Ovule with 8-Nucleate, Polygonum Type of Embryo-Sac:

1. Near the miropylar end is present the egg appara­tus.

2. Egg apparatus consists of an egg and two syner- gids.

3. Near the chalazal end are present three antipodals (Fig. 193).

4. In the centre are present two polar nuclei which ultimately fuse and form a secondary nucleus.

5. Many small vacuoles are present throughout.

Embryology in Plants: Stage # 14.

1. Endosperm is formed because of the fusion of two polar nuclei and one of the male gametes.

2. It has triploid number of chromosomes.

3. It is of following three different types (Fig. 194):

Endosperm nucleus divides many times thus forming many free nuclei, which in the later stages may be separated by walls.

(B) Cellular Type:

In this type all the nuclear divisions are accompanied by the wall forma­tion.

In this type, first the nuclear divisions are accompanied by the wall formation but later on there is no wall formation and nuclei remain free. So it is an intermediate stage between nuclear and cellular.

Embryology in Plants: Stage # 15.

1. Only one cotyledon is present (Fig. 195).

2. Plumule forms the stem and radicle forms the root.

3. Hypocotyle and a small suspensor are also present.

Embryology in Plants: Stage # 16.

1. Two large cotyledones are present.

2. Both the cotyledones cover a small stem apex.

4. Near the suspensor is present the root cap.

5. Central region forms the procambium, which is present in between root cap and stem apex (Fig. 196).

  1. Essay on the Introduction to Embryology
  2. Essay on the Historical Review of Embryology
  3. Essay on the Modern Embryology
  4. Essay on the Scope of Embryology
  5. Essay on Gametogenesis
  6. Essay on the Embryonic Development in Chordates
  7. Essay on the Fertilisation in Chordates
  8. Essay on the Stages of Embryogeny

Essay # 1. Introduction to Embryology:

Embryology (GK., embryon = embryo + logia = discourse) is a study of the origin and development of animals dealing with changes through which a fertilised egg must pass before it assumes the adult state. Fertilisation of an ovum by a spermatozoon results in the formation of a zygote. Development of a single-celled zygote into an adult involves a series of steps or stages resulting in a gradual increase in the complexity of structure.

The stages of embryonic development differ in various chordates, yet the chief phases are basically similar in all. The differences are related primarily to the amount and distribution of yolk present in an egg. The inert yolk or vitellin furnishes nourishment for the developing embryo.

The yolk also influences on the pattern of cleavage, on the morphogenetic movements of the blastomeres during gastrulation and on the type of development, i.e., indirect with larval forms or direct with juvenile stages.

Embryogenesis or embryogeny may be defined as the formation and development of embryos. In fact it includes all the changes by which a fertilised ovum or zygote is transformed into an adult. So long as the developing individual remains in the egg, it is called an embryo. In some lower animals the amount of yolk is less in egg, so that the embryo hatches in earlier stages of development, called a larva.

Usually, it is very different in form and structure from the adult. Examples are caterpillars of insects and tadpoles of frogs. The larva undergoes transformation into the adult by the process of metamorphosis. In higher vertebrates like reptiles, birds and mammals, the eggs are richly supplied with yolk. Their embryos continue development until they attain a form resembling the adult. Examples are chicks of birds and foetuses of mammals.

Essay # 2. Historical Review of Embryology:

Aristotle (384-322 B.C.) was the first Greek philosopher who described the ontogenetic development of chick and many other forms. The doctrines of Aristotle about the development were accepted for a very long time. William Harvey (1578- 1657) and Marcello Malpighi (1628-1694) contributed information on the various stages of the development of chick on the basis of their studies with the help of simple lens. With the discovery of the microscope, Leeuwenhock (1632-1723) described the sperm of man and other mammals.

Some ovists namely Swammerdam and Bonnet advocated an extreme form of preformation theory called encasement or “emboitment” theory. This theory holds that successive generations of individual organisms pre-existed one inside the other in the germ cells of the mother. It was estimated that, as many as, 200 million years of human beings were present, already delineated in the ovaries of Eve.

Such theories of preformation persisted well in the eighteenth century by which time (in 1759) the German investigator Caspar Friedrich Wolff (1733-1794) offered experimental evidence that no preformed embryo existed in the egg of the chicken. He suggested that during embryonic development the organs formed successively in an epigenetic manner.

Wolff advocated that the future embryonic regions of an egg first consist of granules or “globules” (viz., cells or their nuclei) lacking in any arrangement, i.e., these globules do not reveal any resemblance with the form or structure of the future embryo. Only gradually did these “globules” organise into rudiments (germ layers) which, in turn, took on the characteristics of the various organs of the embryo. This method of progressive development from the simpler to the more complex, through the utilisation of building units (globules or cells) is called epigenesis. Today this theory is accepted in a modified form.

K.E. Von Baer (1792-1876), the father of modern embryology, was the first embryologist who first of all, presented the embryological data in a coherent form, made various land mark embryological investigations and made certain very important generalisations. He forwarded the germ layer theory which states that “various structures of the body arise from the same germ layers in different species of animals”.

His most important generalisation is known as Baer’s law which states that “more general features that are common to all the members of a group of animals are, in the embryo, developed earlier than the more special features which distinguish the various members of the group”.

Baer’s law was formulated before the recognition of evolutionary theory, therefore, later on it is reinterpreted in the light of evolutionary theory by Muller and Haeckel (1864) and named as biogenetic law.

In 1824, Prevost and Duman described cleavage or segmentation of the egg. Hertwig in 1875 observed the main events taking place in fertilisation of an egg by a sperm. Von Bender (1883) proved that the male and female sex cells contribute the equal number of chromosomes to the fertilised egg.

During the last days of Nineteenth and early days of Twentieth century, embryologists like Weismann (1883), Endres (1885), Spermann (1901 and 1903) and Morgan (1908) made experimental and analytical investigations and, thus, a new branch of embryology gave way for the initiation of experimental embryology.

In 1883, A. Weismann (1834-1913) suggested convincingly that a child in no way inherits its characters from the bodies of the parents but from the sex cells alone. These germ cells, in turn, acquired their characters directly from the pre-existing germ cells of the same kind.

Wilhelm Roux (1850-1924) in 1881, performed a classical experiment which may be viewed as marking the beginning of the science of experimental embryology. He took a frog’s egg at the two cell stage of cleavage and touched one of the two cells with a hot needle, thus, destroying the nucleus.

He observed that the uninjured cell continued dividing and developed into what he interpreted to be a one-half blastula, a one-half gastrula, and ultimately a one-half embryo.

He, thus, concluded that certain areas of the egg are already destined in the ovary to develop into special region. Thus, the pigmented cytoplasm of animal pole of frog’s unfertilised egg chiefly develops into the head region of the animal, while yolky cytoplasm of vegetal pole of egg forms posterior region.

In 1891, the German Scientist Hans Driesch performed experiment on sea urchin eggs, similar to Roux. He suggested that early cleavages of the egg are equational and have a “quantitative division of homogeneous material”, therefore, the blastomeres have equal potentialities and their fate is determined by their position. Development as observed by Driesch in sea urchin eggs was to be called regulative development and the eggs which were capable of performing such regulative development were called regulative eggs.

Various operative and chemical procedures have been employed in attempts to analyse the developmental processes leading to or involved in the formation of the blastula, the gastrula and the actively swimming larva. Such an experimental approach of T. Boveri (1910), J. Runnstrom (1928), S. Horstadius (1928) and C.M. Child (1936) on sea urchin eggs has contributed a most important theory the gradient theory.

The two gradients are, therefore, the animal gradient with a centre of activity at the animal pole, and the vegetal gradient with a centre of activity at the vegetal pole.

C.M. Child (1936), while recognising the physico-chemical nature of the two gradients, proposed the existence of a single physiological or oxidative metabolic gradient in sea urchin egg.

It was in 1969 and 1972 that Horstadius and Josefsson succeeded in isolating animalising and vegetalising substances from the mature unfertilised egg and early cleavage stages of sea urchin. Arnold (1976) has suggested that the egg cortex by controlling the displacement of membrane receptors and enzyme systems, modulates metabolism in growth, division and cell surface interaction.

In 1924, Spermann and Hilde Mangold published a classical paper providing definitive proof of the organising action of transplanted dorsal lip in the production of secondary embryos, establishing firmly the concept of induction as a basic mechanism in embryonic development. Spermann, thus, has recognised a primary organiser in the form of archenteron in amphibian gastrula and got Nobel Prize of 1935, for such a landmark discovery in experimental embryology.

In modern terms induction can be defined as a type of intercellular communication which is required for differentiation, morphogenesis and maintenance. It is also found that during induction some chemical is transmitted from one tissue to the other and this chemical acts on the genes of the cells being induced to develop into a particular manner. What the substance is has not been still determined, but it appears to be a relatively larger molecule.

Essay # 3. Modern Embryology:

With the discovery of the chromosomes, genes and genetic code, it has become evident that all the properties of any organism are determined by the sequence of the triplets in the DNA molecule. The sequence of the base triplets can directly determine what kind of proteins can be produced by an organism.

All the morphological and physiological manifestations of an organism depend on the assortment of proteins, coded for by the hereditary DNA. The modern embryology is heading towards analytical embryology on the basis of the analysis through molecular biology techniques.

Essay # 4. Scope of Embryology:

Embryology is the most important biological science. It explains the details of the ontogenetic development of an animal from a single fertilised cell. It gives basic information about the physiology, genetics, sex determination, various diseases and organic evolution.

Embryology plays a key role in human welfare. It helps in understanding the causes of congenital malformations, cancer, ageing and in improving the breeds of domestic animals, in controlling pests and vectors of diseases and in the formation of test-tube babies.

Some of the latest phenomena such as teratogenesis, cancer, animal breeding, test-tube babies and cloning, and pest control are most important fields in animal embryology. With the success in cloning experiments of Ian Wilmut (1996) a new concept of cloning without involving germ cells has originated which is useful for the biological resources. The advantages of cloning plants and animals are numerous.

High-yield food plants such as wheat, corn and rice can be selected and abundantly reproduced. Cloning would give animal breeders a tool for exactly reproducing highly desirable animals for example, cloning would make it possible to create 1000 copies of prize dairy cow to help feed growing populations. Endangered species might be saved by cloning numerous replicas of the best of the few remaining individuals.

Essay # 5. Gametogenesis:

The embryogenesis (embryonic development) of a sexually reproducing multicellular animal is prefaced by the gametogenis, i.e., the formation and ripening of two highly dissimilar and specialised sex-cells or gametes, namely a large-sized, non-motile, nutrient filled cell the ovum or egg and a small-sized,motile,sex-cell, the spermatozoon or sperm, both of which unite and give origin to a diploid zygote.

Formation of sex-cell or gametes is termed gametogenesis. It is accompanied by a special type of nuclear division, called meiosis. As a result, the nuclei of gametes formed contain only half or haploid number of chromosomes. When male and female sex-cells (sperms and ova) unite at the time of fertilisation, the resulting cell or zygote again has the full or diploid number of chromosomes.

The production of male germ cells, the sperms or spermatozoa occurs in the male gonads, the testes, by a process called spermatogenesis. Each sperm consists of a head, middle piece and tail. It is preferable to call them sperm cells or simply sperms.

The production of female germ cells, the ova takes place in female gonads, the ovaries, and the process is called oogenesis. The word ‘egg’ is often loosely used for ova or secondary oocytes. It may be reserved for more complex structures such as the hen’s egg which may even contain early embryonic stages.

Essay # 6. Embryonic Development in Chordates:

The stages of embryonic development differ in various chordates, yet the chief phases are basically similar in all. The differences are related primarily to the amount and distribution of yolk present in an egg. The inert yolk or vitellin furnishes nourishment for the developing embryo. The yolk also influences on the pattern of cleavage, on the morphogenetic movements of the blastomeres during gastrulation and on the type of development, i.e., indirect with larval forms or direct with juvenile stages.

The amount of yolk varies in the eggs of different chordates, it determines the size of the egg and the pattern of early development (cleavage and blastulation, etc.). The eggs are classified according to the distribution of yolk they contain into two main types, namely, isolecithal and telolecithal eggs.

A. Isolecithal or homolecithal eggs have very little yolk which is uniformly distributed evenly in the cytoplasm. Such eggs are found in various chordates, e.g., Amphioxus, tunicates and marsupial and eutherian mammals.

B. Telolecithal eggs contain a considerable amount of yolk, which has a polarised distribution. Due to its gravity, it is concentrated more in vegetal hemisphere than that of animal hemisphere. Such polarised distribution of yolk is found in mesolacithal and macrolecithal eggs.

In fact, in macrolecithal eggs, the amount of yolk is so massive that it almost occupies the whole space of the egg, except a small space at the animal pole where the nucleus or germinal vesicle lies in the form of cap over the yolk.

The telolecithal eggs may be either moderately telolecithal (e.g., eggs of Amphibia, Petromyzon and Dipnoi) or highly telolecithal, (e.g., cartilaginous and bony fishes, reptiles, birds and egg-laying mammals). All eggs are enclosed in one or two vitelline membranes.

C. Centrolecithal eggs found in insects and some hydrozoa, contain a large amount of yolk concentrated in the centre of the egg surrounded by thin peripheral layer of active cytoplasm.

Classification of Eggs on the Basis of Amount of Yolk:

1. Microlecithal or oligolecithal eggs are small sized, containing a small amount of yolk. Such eggs are found in Amphioxus, tunicates, and marsupial and eutherian mammals, and also in certain invertebrates such as Hydra and sea urchin.

2. Mesolecithal eggs contain moderate amount of yolk, e.g., annelid worms, molluscs, Petromyzontia, Dipnoi and Amphibia.

3. Macrolecithal, megalecithal or polylecithal eggs contain massive amount of yolk such as eggs of insects, Myxine, elasmobranch fishes, reptiles, birds and prototherian mammals.

The egg or ovum is surrounded by a thin plasma membrane and around it is present a vitelline membrane, which is non-cellular and transparent layer of mucoprotein. It is often much thicker and stronger than the underlying fine plasma membrane. It is differently named in various groups of animals such as chorion in fishes and zona pellucida in reptiles and mammals.

A spermatozoon (Gr., sperma = seed + zoon = animal) or male gamete of vertebrates despite its small size is an exceedingly complex cell. It has a head, a middle piece, and a tail, all of these are contained by a continuous plasma membrane, like other living cell.

The head has a nucleus invested by a thin layer of cytoplasm which projects in front as a pointed acrosome, both performing two basic functions of the sperm – genetic and activating, respectively. The nucleus occupies most of the space of the sperm head. It is enveloped by a typical double nuclear membrane, which lacks the nuclear pores except the lower part.

The nucleus contains only its haploid complement of DNA bound by basic proteins. The nucleus has no nucleolus, RNAs and fluid contents. Acrosome lies anterior to the nucleus and its shape and size varies among different species.

It is also bounded by a unit membrane and contains a number of acid hydrolases, such as acid phosphatase, cathepsin, hyaluronidase, etc. In mammals, it contains acrosomin made of hyaluronidase and acrosin (zona lysin).

It lies behind the nucleus and connected with the head by a narrow neck. Inside the neck, posterior to the nucleus are present two centrioles, both lie at right angles to the other. The anterior or proximal centriole lies in the depression in the posterior surface of nucleus and forms the mitotic spindle in the egg after fertilisation.

The distal centriole or posterior centriole forms the microtubules (axoneme) of the sperm tail (flagellum). It acts as basal body for the flagellum. The distal centriole and the proximal part of the axial filament lie in the middle piece of the spermatozoon. The axial filament of the sperm tail has the same organisation as the axial filament of flagella and cilia.

In middle piece the axial filament is surrounded by numerous well developed mitochondria. In mammals, the mitochondria are joined together forming one continuous body twisted spirally around the axial filament.

However, in other animals, such as in annelid, Hydroides hexagonus, and in sea urchin, Arbacia punctulata, mitochondria are joined in one or more massive clumps, called mitochondrial bodies forming the bulk of the middle piece. They contain all the respiratory enzymes and are extremely active in oxidative phosphorylation.

Around the periphery of middle piece of the sperm is found a condensed layer of cytoplasm that is composed mainly of the microtubules and is called manchette. It also surrounds the posterior part of head of the sperm. At the posterior end of middle piece occurs a dark ring or fibrous thickenings beneath the plasma membrane, forming the boundary between the middle piece and tail. It is called ring centriole or Jensen’s ring.

The tail is a long vibratile flagellum containing an axial filament along its whole length and projecting behind the cytoplasm of the tail as an end piece. Tail has two main parts- principal piece and end piece. The principal piece constitutes most of the tail length, consists of a central core, comprising the axial filament.

Surrounding this core is a microtubular fibrous tail sheath which some time appears as semicircular ribs oriented perpendicular to the long axis of the filament or as helical coils. In human sperms, out of nine coarse fibres found around axial filament, of the tail two coarse fibres are fused with the surrounding ribs so as to form anterior and posterior columns extending throughout the length of the principal piece.

The end piece is merely a short tapering portion of tail containing only the axial filament covered with cytoplasm and plasma membrane.

Spermatozoa are discharged from the body floating in a seminal fluid or semen secreted by the seminiferous tubules and accessory reproductive glands. Spermatozoa are always produced in very large numbers.

Essay # 7. Fertilisation in Chordates:

Fertilisation (L., fertilis = to bear). It is the fusion of two gametes (spermatozoa and ova) and so their nuclei to form a diploid zygote. It activates the egg to form fertilisation membrane outside the egg plasma membrane to start its metabolism and to start its cleavage.

During fertilisation process the jelly coats and egg membranes such as vitelline membrane and plasma membrane secrete the fertilizin and the sperms tip secrete antifertilizin, both interact with each other and sperms, thus, agglutinated. It occurs in the female genital tract.

The membrane of the acrosomal vesicle of the acrosome and the plasma membrane of the sperm breakdown and the severed edges of the two membranes fuse to form an opening through which the contents of the acrosomal vesicle are released.

The inner acrosomal membrane grows into one or many acrosomal tubules which come in contact with the vitelline membrane and plasma membrane. In mammals, the plasma membrane and outer acrosomal membrane break and fuse to give rise to extensive vesiculations and the sperm is possibly phagocytosed by the egg.

The acrosome now releases the lytic enzymes or lysins (acrosomin in sea urchins) which help the sperm to penetrate the egg envelopes by liquefying them locally, without affecting the plasma membrane. In mammals including human females, the sperms first penetrate the multiple layers of follicular cells (granulosa cells) which are held together by an adhesive substance hyaluronic acid.

The acrosome releases the hyaluronidase and proteolytic enzymes for penetrating the follicle cell layers, corona radiata and zona pellucida. Hyaluronidase is supposed to dissolve the cement between cells of corona radiata. Zona lysins or proteolytic acrosomal enzymes are responsible for the passage of the sperms through zona pellucida.

The apical part of sperm plasma membrane (originally the inner acrosomal membrane) extends forward to form an acrosomal tubule. It projects through the egg membranes to reach the egg plasma membrane or oolemma. The shape and size of acrosomal tubule varies with species and is entirely absent in mammals.

The tip of acrosomal tubule fuses with egg plasma membrane, while in mammals sperm come in contact with the egg surface by its lateral aspects. After the fusion, the plasma membrane of the egg and tip of acrosomal tubule dissolve at the point of contact. In teleost fishes acrosome is lacking and so the plasma membrane of sperm head fuses directly with the plasma membrane of ovum.

After fusion of the both the plasma membranes, the plasma membrane of ovum becomes permeable to sodium, potassium and calcium ions. Calcium is essential for the fertilisation process. pH of egg cytoplasm also increases due to inflow of Na + and outflow of H + ions. Within seconds after membranes contact, changes occur in egg cortex.

In bony fishes and frogs, the cortical granules are broken down after sperms’ penetration into the egg cytoplasm and their contents become liquefied and extruded on the surface plasma membrane of the egg. They gradually fill up the perivitelline space in between chorion and egg plasma membrane in bony fishes, and the space between vitelline membrane and egg plasma membrane in frogs.

Thus, fertilisation membrane is formed by the rupture of cortical granules outside the plasma membrane. This is due to the cortical reaction stimulated by the penetrating sperm. Fertilisation membrane blocks the entrance of other living sperms.

The vitelline membrane or chorion does not transform into fertilisation membrane. In some mammals (e.g., man, rabbit and hamster) the cortical granules burst open and release their contents in the space between egg plasma membrane and zona pellucida. Cortical granules are not found in urodele amphibians and, hence, no fertilisation membrane formation occurs.

In most species only one sperm enters the egg and this is called monospermic fertilisation. When many sperms penetrate the single ovum (e.g., in polylecithal eggs of some insects, elasmobranchs, urodeles, reptiles and birds, and also in microlecithal eggs of bryozoans), it is called polyspermic fertilisation. In this case, the genetic material of only one sperm is incorporated in the zygote nucleus, and other sperm nuclei are degenerated.

After the penetration of sperm inside the egg cytoplasm, its nucleus moves inward, swells and its chromatin which is very closely packed becomes finely granular. It finally becomes vesicular and is called male pronucleus. Similarly the egg nucleus after second meiotic division undergoes changes and becomes female pronucleus, which swells, increases in volume and becomes vesicular.

Later on male and female pronuclei fuse together, that is, the nuclear membrane of both pronuclei are broken at the point of contact and their contents unite into one mass, which is finally bounded by a common nuclear envelope, forming a zygote nucleus. This type of fusion of both pronuclei (male and female) is called amphimixis.

Significance of Fertilization:

1. The male and female nucleus possess haploid (n) number of chromosomes. The fertilisation restores the specific parental diploid chromosome number.

2. Fertilisation brings together the chromosomes and genes from two different parents, resulting into a new genetic recombination.

3. Fertilisation activates the egg to undergo cleavage.

Types of Fertilization:

According to place and nature of fluid media, fertilisation is of two types:

A. External Fertilization:

When the fertilisation occurs in the aquatic medium outside the body of female, it is called external fertilisation. Aquatic medium may be sea water or freshwater. In marine animals, sexually mature adults shed eggs and sperms freely into the surrounding water. The sperms and eggs are laid in water in astronomical numbers, and also in close proximity.

B. Internal Fertilization:

In terrestrial forms, where eggs are completely enclosed in impermeable envelopes before being laid such as oviparous animals or where they are retained within maternal body throughout development such as ovo-viviparous and viviparous animals (e.g., elasmobranchs and mammals) the sperms are transmitted internally, i.e., in the body of female, by the intromittent organ of male.

In these forms the fertilisation may occur in the lower portion of oviduct (e.g., urodela) or in the upper portion of the oviduct such as salamanders, reptiles, aves and most mammals. In viviparous fishes such as Gambusia affinis and Heterandria formosa, and certain eutherian mammals such as Ericulus, fertilisation occurs in the ovarian haploid follicles.

The results of fertilisation are:

(a) An activation of the egg to undergo its second maturation division for preparing a haploid female nucleus

(b) An introduction of a centriole by the sperm which divides to form two centrioles, since a centriole is lacking in a mature ovum

(c) A restoration of a diploid number of chromosomes in the zygote

(d) A change in the periphery of the egg which precludes the entry of other sperms

(e) Separation of the vitelline membrane from the egg to allow the zygote to rotate.

The division of an activated egg (zygote) by a series of mitotic cell divisions into a multitude of cells which become the building units of future organism, is called cleavage or segmentation (Ger., kleiben = to cleave). During cleavage, cells do not grow in size and early cleavage divisions occur synchronously, which is lost during late cleavage.

During cleavage, there is no growth in the resulting blastomeres and the total size and volume of the embryo remains the same. The blastomeres do not move so the general shape of the embryo remains the same except the formation of a cavity, the blastocoel in the interior. During cleavage, chemical conversion of reserve food material (yolk, glycogen and ribonucleotides) into active cytoplasm takes place.

Thus, a steady increase of respiration occurs throughout cleavage. During cleavage, nucleo-cytoplasmic ratio in cells is reduced, which permits the cells to be more metabolically active, because such nuclei have less cytoplasm to control. Thus, the cleavage converts the egg into a compact mass of cells or blastomeres called morula.

The type of cleavage taking place depends largely on the amount of yolk present.

Following types of cleavages occur:

a. Holoblastic or Total Cleavage:

In this type of cleavage, the entire egg divides by each cleavage furrow.

It is subdivided into two types:

(i) Complete or equal holoblastic cleavage occurs in microlecithal and isolecithal eggs, the entire zygote divides completely to produce a number of almost equal-sized cells, e.g., eutherian mammals, Amphioxus, tunicates.

(ii) Unequal holoblastic cleavage occurs in mesolecithal and telolecithal eggs, the zygote divides completely to form unequal-sized blastomeres, i.e., small-sized cells towards the animal pole which has almost no yolk, larger cells towards the yolky vegetal pole, e.g., cyclostomes, elasmobranchs, Dipnoi and Amphibia.

b. Meroblastic or Incomplete Cleavage:

This occurs in polylecithal eggs in which only the small germinal disc lying at the animal pole consisting of clear cytoplasm and a nucleus, undergoes a series of incomplete divisions forming an area of cells at the animal pole, the large yolky portion beneath the germinal disc remains unsegmented, e.g., toleosts, reptiles, birds and egg-laying mammals. Here the germinal disc is of disc-shape, so the cleavage is also called discoidal.

c. Superficial Cleavage:

This type of incomplete cleavage is found in centrolecithal eggs, e.g., insects and many arthropods. The nucleus lying in the centre of the egg yolk surrounded by an island of cytoplasm undergoes cleavage, and each nuclei is surrounded by small amount of cytoplasm.

They later move towards the periphery in the peripheral cytoplasm. Here their cytoplasm fuses with the peripheral cytoplasm. Later the peripheral cytoplasm becomes subdivided by furrows extending inward from the surface, thus, a layer of peripheral or superficial cells is formed which surrounds the central undivided yolk.

The pattern of cleavage due to organisation of egg may be of following types:

i. Radial Cleavage:

When successive cleavages extend through the egg, at right angles to one another and the resulting blastomeres become symmetrically arranged around the polar axis. Such type of cleavage is called radial cleavage, and is found in echinoderms (e.g., Synapta and Paracentrotus, etc.).

ii. Biradial Cleavage:

When first three cleavage planes are not arranged at right angles to each other, it is called biradial cleavage, e.g., Acoela like Polychoerus and Ctenophora.

iii. Spiral Cleavage:

The rotational movement of cells around the egg axis during cleavage is due to spiral cleavage. The spiral cleavage results due to oblique positions of mitotic spindles in the blastomeres. Thus, it is also called oblique cleavage. In successive cleavages, the rotational movements alternate in clockwise direction or anticlockwise direction. It is found in Turbellaria, Nematoda, Rotifera, Annelida and molluscs except cephalopods.

iv. Bilateral Cleavage:

In this type of cleavage the mitotic spindles and cleavage planes remain bilaterally arranged with reference to a plane of symmetry which coincides with the median plane of the embryo. It is found in Tunicata, Amphioxus, Amphibia and higher mammals.

v. Determinate and Indeterminate Cleavage:

The cleavage in nematodes is of a special type of bilateral cleavage in which definite blastomeres give rise to specific parts of the embryo. This type of cleavage is called determinate or mosaic cleavage. In vertebrates, the plane of cleavage is less rigid, the cleavage pattern has no definite relation to the embryo.

This type of cleavage is called indeterminate or regulative and is found in echinoderms, Balanoglossus, coelenterates and amphibians. A first cleavage blastomere of a sea urchin or an amphibian or a mammal, when isolated can alter its usual destiny and develop into a perfect (but small) embryo. Similarly, when two fertilised eggs, made to adhere like a two-cell stage, they produce a single giant embryo. This is regulative development.

Essay # 8. Stages of Embryogeny:

During early cleavages, the blastomeres tend to assume a spherical shape and their mutual pressure flattens the surfaces of the blastomeres in contact with each other, but their free surfaces remain spherical.

Thus, cleavage process develops a multicellular body with loosely arranged blastomeres with in fertilisation membrane, called morula (Latin word for mulberry) resembling mulberry, e.g., amphibian and coelenterates. In macrolecithal eggs, morula is a cellular flattened disc at the animal pole.

As cleavage proceeds the cells increase in number but become smaller. The cells withdraw from the centre and arrange themselves towards the surface to form a true epithelium, which may be single cell thick as in Amphioxus, echinoderms, etc., or many cell thick as in most vertebrates.

Due to rearrangement of cells to form the epithelium or blastoderm a fluid-filled space or blastocoel or segmentation cavity is formed. This stage is called blastula and the process of formation is called blastulation.

Types of Blastulae:

i. Coeloblastula:

It is in the form of a hollow sphere formed of a single layer of blastoderm and the blastocoel is filled with mucopolysaccharides. Examples, echinoderms and Amphioxus.

ii. Stereoblastula:

In spirally cleaving eggs of annelids, molluscs, nemerteans and some planarians, blastula is solid, having no blastocoelic cavity. In them micromeres accumulate as cluster of cells over macromeres of vegetal hemisphere.

iii. Periblastula or Superficial Blastula:

In superficially cleaving eggs of insects, the blastocoelic cavity is not found. The central yolk is surrounded by peripherally arranged cells.

iv. Discoblastula:

In large yolky eggs of fishes, reptiles and birds discoblastula is found. It is a small multilayered flat disc separated from the yolk by a narrow subgerminal cavity.

v. Amphiblastula:

It is found in amphibians. The blastula contains micromeres in the animal hemisphere and macromeres in the vegetal hemisphere, and a small fluid-filled eccentric blastocoel in the animal hemisphere.

It is found in mammals. Cleavage is regular and a small cavity appears inside the dividing cells, which gradually increases in volume. This is the blastocoel. The cells surrounding the blastocoel are the trophoblast cells or nutritive cells and an inner cell mass of formative cells displaced to one pole of the blastocyst.

A rearrangement of the cells of the blastula occurs in which some cells are differentiated and come to lie inside, while the other cells enclose them, this stage is gastrula and the processes converting the blastula into a gastrula are known as gastrulation. Gastrulation process (morphogenetic movements of cells) converts a simple one-layered blastula into a two-layered (e.g., Amphioxus) or a three-layered (e.g., all vertebrates) gastrula (Gr., gaster = stomach or gut).

The single layer of blastula is called blastoderm, ectoblast or proctoderm. The three layers (ectoderm, mesoderm and endoderm) are called germinal layers. The blastocoel is generally obliterated and the inner layer of cells (endoderm) of the gastrula encloses a new cavity called archenteron which opens on one side to the exterior by a blastopore. During gastrulation embryo acquires antero-posterior polarity and bilateral symmetry.

After gastrulation the continuous masses of cells of the three germ layers split up into smaller groups of cells, called primary organ rudiments, each of which produces a certain organ or part of the animal body. These organ rudiments further develop simple organs and parts and, thus, embryo develops into either larval form or a miniature adult. Thus, the formation of organs from the germ layers is called organogenesis.

Derivatives of Germ Layers:

The ectoderm forms a neural tube which gives rise to the brain, spinal cord, and nerves. The forebrain forms the retina, and part of the iris. The ectoderm forms the lens, conjunctiva, and a part of the cornea, the membranous labyrinth and the lining of the nose.

In fishes and aquatic amphibians, the sensory parts of the lateral line system arise from the ectoderm. The neural crest cells lying between the outer ectoderm and on both sides of the neural tube give rise to ganglia of the spinal nerves and autonomic nervous system, the neurilemma of peripheral nerves, chromatophores of the skin, some neural crest cells give rise to mesenchyme which produces the visceral arches, and some neural crest cells wander inwards and form the suprarenal gland near the kidneys, but in mammals they form the medulla of adrenal glands.

Supporting part of the central nervous system called neuroglia is derived from the neural tube. The ectoderm forms the epidermis of the skin and many epidermal derivatives, such as skin glands, epidermal scales, nails, claws, hoofs, horns, feathers and hairs.

Ectodermal invaginations form the stomodaeum and proctodaeum which meets the archenteron, the ectoderm of the stomodaeum forms the lining of the mouth and lips, glands of buccal cavity, enamel of teeth, covering of tongue, and anterior and intermediate lobes of the pituitary gland (the posterior lobe of the pituitary is formed from the forebrain).

The ectoderm of proctodaeum forms the lining of the cloaca and some anal and cloacal glands. From the dorsal side of the forebrain one or two evaginations take place, the anterior one is an eye-like parietal body which is present in lower forms only, the posterior one is the pineal body found in all vertebrates.

The archenteron is formed from endoderm, it becomes the lining of the adult alimentary canal, except in the buccal cavity and cloaca. Two outgrowths of the digestive tract form the liver and pancreas, the endoderm forming their epithelial lining only, and also of the gall bladder and bile duct.

From the pharynx, the endoderm pushes out to form several pairs of pharyngeal pouches. In cyclostomes, fishes, and amphibians, the pharyngeal pouches meet the ectoderm to form gill-clefts which open to the exterior. In amniotes, the pharyngeal pouches do not perforate to the exterior, in tetrapoda, the first pair is modified to form the cavity of the middle ear and Eustachian tube.

An evagination of the pharynx along with some pharyngeal pouches forms a thyroid gland. In air-breathing vertebrates the endoderm of pharynx forms the lining of the larynx, trachea, and lungs. Endoderm of some pharyngeal pouches form part of the tonsils, thymus, parathyroid glands and ultimobranchial bodies.

In amniotes, the archenteron forms a large bag, the allantois, its lining is endodermal. Endoderm cells of the archenteron grow out in embryos developing from polylecithal eggs to form the lining of the yolk sac to enclose the yolk, the yolk sac disappears in the adult. It must be noted that organs arising from the archenteron have only their lining and epithelial cells formed from endoderm, the supporting tissues of these organs are mesodermal.

The mesoderm becomes differentiated into three major parts- a dorsal epimere which is segmented, a median mesomere, and a ventral hypomere. Further development of mesoderm forms mesenchyme which is not a germinal layer but a primitive kind of embryonic connective tissue with branching cells forming a network. Nearly all mesenchyme comes from mesoderm though other germinal layers may also contribute to its formation.

The epimere is differentiated into sclerotome, dermatome, and myotome. The middle parts of epimeres form mesenchyme which gathers around the neural tube and notochord to form the sclerotome. The mesenchymatous sclerotome forms the vertebral column.

The dermatome transforms into mesenchyme which migrates to lie below the ectoderm and gives rise to the dermis of the skin. The remaining portion of the epimere is called myotome, the adjacent myotomes are separated by myocommata which are connective tissue partitions. The myotomes of the two sides grow down between the skin and somatic layer of mesoderm to meet midventrally, they give rise (with some exceptions) to voluntary muscles of the body and body wall.

(ii) Mesomere forms the urogenital organs and their ducts, the terminal parts of these ducts may have ectodermal or endodermal lining.

(iii) Hypomere splits into somatic and splanchnic layers of mesoderm enclosing the coelom. The splanchnic layer forms mesenchyme which gives rise to involuntary muscles and connective tissue of the alimentary canal and of the organs formed as outgrowths of the archenteron.

The splanchnic mesoderm forms the heart. The remainder of the splanchnic mesoderm together with the somatic mesoderm forms the lining of the coelom, pericardium and lung pleura or peritoneum. Splanchnic mesoderm also forms the mesenteries and omenta.

(iv) Mesenchyme (Gk., mesos = middle + enchyma = infusion) gives rise to all the connective tissue, blood vessels, lymph vessels, lymph nodes, blood corpuscles, all involuntary muscles, parts of the eye, dentine of teeth, and to cartilage and bones of the entire skeleton, except the vertebral column. It is claimed that voluntary muscles of limbs are formed from mesenchyme and not from myotomes.

4 Main Developmental Organs in Vertebrates | Embryology

The first important morphogenetic change following gastrulation is the deve­lopment of the central nervous system. The central nervous system starts as a simple tubular nerve tube which, in course of development, transforms into brain, spinal cord and their associated structures.

The morphogenetic processes involved in this process are designated as neuralisation. It includes the separation of neural materials from the embryonic ectoderm, their migration inward to form a hollow nerve tube together with the segregation of neural crest cells. The nerve tube diff­erentiates into the brain and spinal cord, while neural crest cells develop into neuroblasts and many other structures.

Methods of Neuralisation:

Neurali­sation occurs by two ways in different ver­tebrates.

(a) Thickened Keel Method:

In teleost, ganoid fishes and cyclostomes the neural materials become aggregated to form a thickened keel or ridge extending along the mid-dorsal axis of the body. This ridge sepa­rates itself from the overlying ectoderm and develops a lumen within to form a tube.

It occurs in most of the vertebrates where neural cells be­come aggregated to form a neural plate. This plate folds inward to form neural groove. The neural groove transforms into a neural tube which sinks from the over­lying ectoderm.

Events in Neural Morphogenesis:

After the completion of gastrulation, the ectoderm of the future dorsal side of the developing embryo tends to condense to form a thick and compact neural plate with elevated margins. This thickened part is called the neural (or medullary) plate or neural placode.

The plate is formed by two simultaneous processes:

(a) Elongation of prospective neural cells in the direction perpendicular to the surface of the deve­loping embryo, and

(b) Shrinking the ex­posed surfaces both dorsally and ventrally.

The neural plate is pear-shaped, i.e. it is broader at the anterior part but gradually narrows towards the posterior end. This particular shape of the neural plate is crucial for shaping the future structures. The shaping of the neural plate is resulted as the consequence of regional differences in the cell contration. Fig. 5.25 relates the stages of neural morphogenesis in frog.

A depression appears along the entire length of the neural plate which folds downward to form a neufal or medullary groove. The formation of the neural groove is associated with the median and dorsal movement of the ectodermal layer attached to the lateral edges of the neural plate.

Thus the raised or folded margin of the neural groove is called the neural fold. The downward movement of the neural plate to form the neural groove depends largely on the lateral shifting of somatic mesoderm from the notochordal area to accommodate the invaginating neural groove. The lateral neural folds rise and meet along the middle line. This union begins from the anterior end and runs posteriorly.

With the union-of the folds, the outer ectodermal layers become conti­nuous and the inner nervous layer, after fusion with the corresponding part, forms a tube and separates itself from the upper ectodermal layer. This tube-like structure is called the neural tube.

The cavity of the neural tube is called the neurocoel which is broader at the anterior end and opens to the exterior through an opening called the neuropore. The neuropore ultimately closes at the later stage of development.

Associated with the formation of neural tube neural crest cells become segregated on the two sides of the neural tube. These neural crest cells lie as two longitudinal strips of cells, one on each dorsal side of the neural tube.

Neural Crest and its Fate:

At the corners of the fusing neural fold during brain formation, groups of neural crest cells become detached to occupy a position over the neural tube. In course of develop­ment these cells leave their position and migrate to other parts of the embryo.

These cells are versatile in their developmental fate and develop neuroblasts of the spinal and sympathetic ganglia, Schwann sheath cells producing the myelin sheath and neurilemma of the nerve fibres, melanoblasts, chromaffin tissue of adrenal me­dulla, meninges, cartilages of the jaw, etc. Weston (1963) has shown the migration of neural crest cells.

The neural tube and neural crest cells labelled with radio­active isotopes are excised from the trunk of a developing chick embryo and trans­planted to a normal (non-labelled) host in place of its counterparts.

It has been shown that the neural crest cells migrate along two ways:

(i) Dorsolaterally along the skin and

(ii) Ventrolaterally in relation to the neural tube.

Structural Differentiation of the Neural Tube:

The differentiation of the neural tube into the brain and spinal cord depends upon many intrinsic and extrinsic factors. The anterior part of the neural tube transforms into the brain while the posterior narrow part becomes elongated to form the spinal cord.

The broad anterior part is demarcat­ed from the narrow posterior part by isthmus. Remarkable changes occur in the anterior part during its conversion into the brain.

This is caused by:

(a) Unequal thickening of the neural tube wall,

(b) In­vaginations or evaginations of the wall and

(c) Various types of bending or folding (flexure formation).

Immediately after the formation of the neural tube, the anterior part swells up and two constrictions develop to divide the anterior part into three general regions: Prosencephalon, Mesencephalon and Rhom­bencephalon.

In course of development, pro­sencephalon and rhombencephalon be­come further subdivided thus giving rise to five parts: Telencephalon, Diencephalon, Mesencephalon, Metencephalon and Myelen- cephalon (Fig. 5.26).

Many factors are res­ponsible in brain morphogenesis. Diff­erential growth and intraventricular pres­sure are regarded to be the important morphogenetic factors in brain develop­ment, especially in flexure formation. Fig. 5.26 relates the development of flexures and different regions of the brain.

Histogenesis in Brain Development:

The early neural tube is fairly uniform in structure. The walls are composed of neural epithelial cells which eventually differentiate into: (1) neuroblasts and (2) spongioblasts. The neuroblasts develop into nerve cells and fibres while the spon­gioblasts give origin to ependymal and neuroglial cells.

The neural epithelium is composed of pseudostratified columnar epithelial cells which form the primitive ependymal layer or matrix layer. Gradually the cells of the matrix layer migrate to cach lateral side to form a cellular layer called the mantle layer.

And lateral to the matle layer lies a cell-free marginal layer. The cells of pri­mitive ependymal layer are usually called the germinal cells, some of which after a day or two following the closure of neural groove, develop neuroblasts and migrate first to the mantle layer (Fig. 5.27).

In the mantle layer, the cells differentiate into

(a) Neuroblasts and neurons and

(b) Spongioblasts and neuroglial cells.

The neuroglial cells give rise to astrocytes and oligodendrocytes. The neuroblasts do not remain evenly distributed but are aggre­gated into clusters. From mature neuro­blasts, nerve cells and fibres grow out in a distinct pattern and turn the brain into a ‘working unit’.

Development of Nerve Cells and Fibres:

Nerve cells originate from neuro­blasts which develop from the neural tube, neural crests and cranial placodes. The actual stages of conversion of a small (neuroblast to a large cell-body of a nerve cell can be seen in tissue culture method reported first by Harrison in 1907. As in Fig. 5.28, a small fragment of neural tube is transplanted in a blood clot and kept sealed in a moist chamber.

Dissociation and dispersion of cells are the first observ­able events in tissue culture. The origin of nerve fibres is the most notable event in this process of conversion. Three theories are extant on this particular issue.

(a) Cell-chain theory. This theory relates that the fibre is laid down by chains of cells which surround the nerve fibre.

(b) Plasmodism theory. According to this theory the nerve fibre is laid down on preformed pro­toplasmic bridge.

(c) Outgrowth theory. The theory advocates that the fibre is formed as an outgrowth of a single neuroblast.

The tissue culture experiment gives sup­port to the last concept and setties the long standing controversy regarding the issue. At the beginning, a thin strand of protoplasm emerges as outgrowth from one side of the neuroblast.

This outgrowth becomes amoeboid and creeps along the solid object. The outgrowth has developed a growth cone at the terminal end which may branch to form two or more growth cones. The growth of nerve fibre exhibits streotropism, i.e. it moves along solid object.

Causal Analyses in Brain Morpho­genesis:

In the entire process of nervous system formation, a number of inductive events occur. In the amphibian eggs, the dorsal lip of blastopore acts as primary organizer to induce the inward moving cells to form chordamesoderm which in turn induces the dorsal ectoderm to be neuralised. The formation of the neural tube is also guided by the influence of regionally specific inductions.

The neural plate at the beginning is an oval, flattened plate and is formed by the ectodermal cells which have come from lateral regions to the dorsal side. The neural plate elongates rapidly, which is caused by the movement of cells. The cells first move towards the middle and then run in two directions: anteriorly and pos­teriorly.

The transformation of neural plate to neural tube which is called neurulation is also known to occur in vitro. It begins with a depression in the centre and curving of the edges which fuse together to form the tube. To search the motive force behind the formation of tube, the be­haviour of cells in the centre and periphery is intimately studied.

Certain suggestions, like differential water uptake, diff­erential cell divisions have been negativated. It is now claimed that elongation of the plate is due to migration of cells but curvature is caused by changes in the cell adhesion.

In further development, the anterior part of the tube swells up considerably to form brain vesicles. Considerable amounts of cell division and cell movement occur during the process. The different parts of the brain in course of its development in­duce the formation of structures like optic, auditory and nasal placodes on the outer ectodermal covering.

It must be remembered that mesoder­mal cells which immediately remain around neural tube are believed to play most important role in the epigenetic pro­fess.

The formation of brain establishes:

(a) many histological features remain deter­mined at neural plate stage and

(b) all the cells do not transform into neural element at the same time. On the contrary a gra­dient exists in the anterior-posterior plane.

Developmental Organ # 2. Eye:

The early stages of eye development follow a generalised pattern in all verte­brates and the details of eye morpho­genesis in chick will give an idea of the process in general. The eye is a very com­plicated structural unit. The development of eye reveals the incorporation of different tissues which follow an orderly fashion to give the geometry of pattern.

Because of the fact, the incidences involved in eye morphogenesis are regarded as a perfect model to explore the general problems of embryology.

The development of eye is discussed under three steps:

(a) Development of sensory areas.

(c) Development of associated structures.

Development of Sensory Areas:

Formation of Optic Placode and Optic Vesicle:

The primordial eye rudiment lies at the very anterior end of neural plate in the form of two closely placed oval areas, one on either side of the middle line. They are lined below by mesoderm. In course of the formation of brain, these two lateral sides of the forebrain, which are destined to be the future diencephalon, become thickened.

These parts are known as optic placodes. These two placodes extend laterally as small blunt bulgings, which become known as optic vesicles (Fig. 5.29). The vesicles elongate through the loose mesenchyme to­wards the epidermal covering and remain connected with the brain by a narrow stalk called optic stalk.

Formation of Optic Cup:

As the optic vesicle touches the ectoderm, the ectoderm cells elongate perpendicularly to the re­gion of contact to form the lens placode which invaginates to form lens vesicle. With the invagination of lens placode to form lens vesicle, the optic vesicle reverses its outward bulging and turns inwards to form the optic cup to accommodate the lens. The optic cup is double-walled.

Such inpushing takes place asymmetrically and continues obliquely into the optic stalk. Near the optic stalk, a slit is left in the ven­tral side which is called choroid fissure. This fissure acts as an outlet for optic nerve (Fig. 5.30). The blood vessels also find an impasse into the optic cup.

The outer wall of the optic cup remains thin and gives rise to pigmented retina while the inner wall (neural retina) becomes greatly thickened and elaborated to transform into light sensitive retina.

Formation of Retina and Optic Nerves:

The inner lining of the optic cup transforms into light sensitive retina, which in turn differentiates into seven layers of nervous elements. The neural retina gives rise to rods and cones and some other types of cells with which the visual cells synapse.

The histogenesis of retina is divided into three phases:

(i) A phase of cell multiplica­tion,

(ii) A phase of cellular readjustment and

(iii) A phase of final differentiation.

Fig. 5.31. gives the details of development of retinal cells. Rods and cones are arranged in the outermost part, i.e., towards the pigmented layer of the optic cup. The position of visual cells is due to the migration and stratification of neural layer of the retina. The fibres of rods and cones unite with the fibres of the ganglionated layer of the retina. These fibres converge towards the optic stalk to form the optic nerve.

The region of the outer ectoderm which comes in contact with the optic vesicle thickens and is known as lens placode. The placode invaginates to form a lens pit or lens cup. The two ends of the lens cup unite and remain within the space between-optic cup and outer ecto­derm. It is then called lens vesicle.

The inner cells of the vesicle transform into lens fibres and the cell layer next to ectoderm forms the epithelium of the lens. At about 96 hours in chick embryo, the lens cavity becomes reduced and the cells of the median wall of the lens become elongated to obliterate the lens cavity.

The cyto­plasm of the cells becomes clear and these cells transform into lens fibres. The extreme elongation of the cells is evidenced by the placement of their nuclei in the equitorial region of the lens and the cells are stretched extending from one surface of the lens to the other. A lens fibre may reach 10 mm as seen in man.

Development of Associated Structures:

The optic cup and optic stalk become invested with a layer of mesenchyme which later forms an outer densely fibrous layer called sclera and an inner pigmented and richly vascularised layer called choroid.

Conjunctiva, Cornea and Aquous Humor:

After the detachment of the lens the ecto­dermal epithelium forms a cell-free lining. Along the inner border of the ectodermal epithelium lies the mesenchyme, which forms the sclera. The inter epidermal layer is called conjunctiva and inner mesenchymal continuation of sclera is known as cornea. The transparency of cornea is vital for the admission of light into the cavity of eye.

The cornea is composed of an outer epi­thelial layer (derived from ectoderm) and a postepithelial stroma (derived from im­migrating mesenchymal cells). In the stroma, layers of collagen fibres accumu­late parallel to the surface. The stioma undergoes significant biochemical changes and undergoes dehydration.

Due to loss of water and suppression of pigmentation, the cornea attains perfect transparency. A space develops between lens and cornea, which is called anterior chamber. A watery fluid, called aqueous humor accumulates within the space.

The choroid layer extends in front of the lens to form a circular-curtain known as iris, which has a hole in the centre called pupil. The iris is pigmented and possesses papillary muscles which regu­late the diameter of the pupil. The iris develops from the pigmented retina.

The pigmented outer layer of the optic cup together with mesen­chymal elements extends in front to form ciliary muscles or ciliary bodies.

The cavity between lens and retina is known as posterior chamber. It becomes filled with a gelatinous matrix called vitreous humor.

Several muscles arc formed by the condensation of outer head mesenchyme. These muscles are involved in rotating the eye-ball within the orbit.

The outermost epidermal layer in front of the eye becomes skin. It splits into two halves to form the eyelids. In different vertebrates the shape of eyelids varies.

Causal Analysis of Eye Development:

The foregoing description reveals that the development of eye is a phasic pheno­menon where different component parts appear in sequential order to establish a harmonious functional unit.

A resume of the total events shows that the optic vesic­les emerging from the forebrain make inti­mate contact with the presumptive lens ectoderm to induce lens formation. The lens vesicle induces the optic vesicle to form optic cup and its subsequent differentiation.

The optic cup-lens complex induces the overlying ectoderm with some mesenchyme to form the cornea. Thus a reciprocity of induction occurs in eye morphogenesis—and this type of induction is called the synergistic induction. Fig. 5.32 gives, the schematic representation of the participation of different tissues and the inductive phenomena in the development of eye.

Events of Eye Development:

Events of eye development may be divided into three phases:

(b) Phase of cell differentiation and axiation and

(c) Phase of mechanical tension.

The work carried by large number of workers have revealed that no organ is formed if there is any disturbance in the first phase. But at the same time only the occurrence of first phase cannot form the organ.

The disturbance in the second phase, i.e. phase of cell differentiation, produces numerous deviations in the diff­erent rudiments of eye. The third phase creates the form and size of the organ and its abnormality affects them considerably.

Some of the important findings to ex­plain the mechanism of eye formation are discussed below:

(i) The lateral extension of the optic vesicles to reach the ectodermal layer is caused by the pressure exerted by the intraventricular fluid of the brain.

(ii) The loose surrounding mesenchyme of the pri­mordial eye rudiment plays important part. Experimental evidences suggest that the mesoderm first contributes most actively to the development of the eye and then exhibits its formation in the middle of the brain.

(iii) In the primary eye rudiment at the beginning, the different layers have the capacity to undergo mutual transforma­tion.

The inner wall of the optic cup trans­forms into the neural retina while the outer wall develops into the pigmented retina., If the position of these layers is reserved, the original inner layer may form neural retina and vice versa.

The trans­formation of the inner wall of the optic cup to form neural retina is caused by the inductive influence of the lens vesicle. The outer layer is converted into pigmented retina by the influence of mesenchyme.

(iv) The proportion of pigmented retina and neural retina depends on the extent of tension produced by the fluid which accumulated inside.

(v) The formation of cornea depends upon geometric distribu­tion of different layers and the mechanical tension of the eye.

Developmental Organ # 3. Heart:

The formation of heart (cardiogenesis) in vertebrates is one of the most dynamic events in embryonic development. The heart is a mesodermal organ, which diff­erentiates initially from the ventral edges of the lateral plate mesoderm.

Primarily the cardiac primordia are paired which, however, become fused to form a single organ. The process of cardiogenesis in different vertebrate forms is essentially similar. The events of the development of heart in the chick embryo are discussed below.

Localisation of Cardiac Primordial:

The localisation of heart-forming cells occurs at the onset of gastrulation in the embryos of all vertebrates. Vogt (1929), by using vital staining technique, has localis­ed the prospective heart cells in amphibian embryo. Butler (1935) and Spratt (1942) have shown that heart-forming cells are widespread in the blastodisc of chick (Fig. 5.33A).

With the movement of cells of the epiblast to form the primitive streak, heart- forming cells become restricted to the epiblastic region anterior to the develop­ing primitive streak.

With the migration of epiblastic cells to form the mesodermal layer, the heart-forming cells become con­centrated about Hensen’s node. The heart- forming cells, then, migrate to join the mesoderm and move laterally. When the definitive primitive streak is formed, the heart-forming cells take lateral position as paired cardiac primordia.

Each primordium is capable of developing a whole heart. If the paired primordia are prevent­ed from fusion, two independently beating hearts (‘cardiac bifida’) will result in an embryo.

Before the formation of heart, the pre­sumptive heart-forming cells acquire spe­cific biochemical characteristics from their neighbours and have an inherent capacity of undergoing self-differentiation. This is attested by the fact that these bilaterally located cardiac primordia, when trans­planted into an indifferent location, are capable of differentiating into cardiac tissue.

The presumptive heart-forming cells are rich in glycogen which is retained throughout its differentiation. So the heart-forming cells become different from other cells by having high glycolytic metabolism.

Stages of Heart Formation:

In course of development of heart, the paired cardiac primordia come together in the midventral line. This is brought about by the action of four types of morphogene­tic movements. These are:

(a) Folding Movements of Ectoderm and Endoderm:

This movement of the endo­derm to develop into crescentic pouch of the anterior intestinal portal and early foregut is of great importance. The pre­sumptive heart-forming cells use the endo­dermal layer as the substratum for their migratory activity. The folding move­ments bring the paired cardiac primordia together in the midline to develop into an unpaired median tube.

(b) Formation of Amniocardiac Vesicles:

This process of development of embryo­nic coelom or amniocardiac vesicle is also important in heart formation. With the formation of head fold and initiation of foregut, the lateral plate mesoderm in the region of cardiac primordia splits to form a dorsal layer (somatic mesoderm) and a ventral layer (splanchnic mesoderm).

The coelomic space thus enclosed by these two layers is called the early pericardial or amniocardiac vesicles. With the separa­tion of the somatic and splanchnic meso­derm, all the presumptive heart-forming cells move ventrally in the splanchnic mesodermal layer. Because of this reason, this thickened crescentic splanchnic meso­derm is called by Mollier (1906) as the ‘cardiogenic plate’.

(c) Cell Movement in the Splanchnic Meso­derm and the Subsequent Emigration of the Mesodermal (Splanchnic) Cells:

Prior to the formation of coelomic space, the precardial mesodermal reticulum consists of a homo­geneous loose meshwork of stellate mesen­chyme. Within this meshwork small clus­ters of tightly packed cells are present which move actively from their lateral position to form the tubular heart.

(d) Formation of Angioblasts:

With the development of cardiogenic plate, angio­blasts are formed in the region of the original amniocardiac vesicle. The conversion of the precardial cells to angio­blasts is the first sign of histological diff­erentiation in cardiogenesis.

These cells migrate either singly or in small clusters out of the mesoderm and form a loose layer (vascular layer of Pander) in the meso-endo- dermal space. This layer forms the endo­cardium of the heart and in the posterior region it produces the blood islands. These islands produce the endothelium of the remaining vasculature, erythroblasts and blood plasma.

Formation of Primitive Tubular Heart:

As stated earlier, the primordial endo­cardial cells begin to differentiate inde­pendently as a pair of delicate tubular hearts. These paired tubular hearts are arranged on either side of the anterior intestinal portal (the opening from the yolk into the foregut).

The folding move­ment of the ectoderm and endoderm to form the head fold and foregut, causes the migration of the paired tubular hearts together when they fuse to form an un­paired tubular heart tube.

The paired rudiments meet and fuse when the foregut is separated from the yolk sac. This process begins at 7 to 8 somite stages in chick and is completed when the embryo comes to 20- somite stage. Each of the paired rudiments has an inner endothelial lining (endocar­dium) and the outer is the epimyocardium.

Simultaneously with the migration of splanchnic mesoderm anteromedially and separation from the folding endoderm, it becomes thickened to form paired epimyocardia. This layer develops later into the thick myocardium and a thin nonmuscular epicardium (or visceral pericardium).

Fig. 5.34 shows the origin and subsequent fusion of paired cardiac primordia during cardio­genesis. The epimyocardium remains atta­ched ventrally by ventral mesocardium and dorsally by dorsal mesocardium. Both these mesocardia disappear subsequently.

The fusion of the heart tubes begins at the anterior end and extends gradually to the posterior sides. Fusion starts in the re­gion of the future ventricle and the auricle is still represented by double tubes. Then gradual union occurs in anteroposterior direction and the process of fusion is com­pleted when the embryo becomes 20- somite stage in chick.

Inductive Relationship during Cardio-­Genesis:

Many embryologists claim that the deve­lopment of heart is intimately related to the developing endoderm. In amphibian embryo, the removal of endoderm causes the failure of heart formation. But in the development of chick, the removal of endo­derm does not prevent normal cardiogene­sis.

Many embryo­logists have claimed that the migration of cardiac cells and the folding movement of the endoderm is independent processes, normal cardiogenesis is not hampered if such relationship is disturbed. But experi­mental evidences on this line are unsatis­factory to ascertain the actual role of endo­derm on the precardial mesodermal cells.

Histological Differentiation in Cardio­genesis:

The differentiation of angioblasts from the splanchnic mesoderm and the trans­formation of the splanchnic mesoderm itself to form the myocardium and epimyocardial mantle are the first indication of histogenesis in heart development.

When the first tubular heart tube is pro­duced, the space between the endocardium and myocardium becomes filled by ‘car­diac jelly’. It is a thick gelatinous mass containing aldehyde, acid mucopolysaccharides. Many cells from the endocardial and myocardial layers migrate into this gelatinous layer to form a loose meshwork of stellate cells which characterise the early heart tube.

Conflicting views exist as regards the histological nature of heart tissue, nature of fibrillogenesis, the nature of myofibril and the nature of intercalated disc. Elec­tron-microscopic and tissue culture studies have revealed that heart tissue is syncytial in nature and the intercalated disc consists of a pair of apposed cell membranes.

The region is covered with electron-dense gra­nules. The myofibrils do not cross the inter­calated discs and there is no protoplasmic continuity across the apposed membranes.

The early tubular heart consists of endo­cardium and myocardium. The endocar­dium is composed of a single layer of flattened and granulated cells, while the myocardium is two or three cells deep. Subsequently in course of development the myocardium thickens by mitotic acti­vity.

Myoblasts, composing the myocar­dium, contain granular materials and scat­tered loose myofilaments which become grouped to form striated myofibrils shortly before the pulsating of the heart. Mitotic activity is very high in early stage which declines to zero as cardiogenesis is com­pleted.

Structural Differentiation in Cardio­genesis:

One of the important factors, which causes the regional differentiation of heart is the rapid elongation of the primitive heart tube within the lass-rapidly growing pericardial space. The heart is a straight tube when it is formed and does not show any sign of subdivision into chambers.

In course of development, the tube becomes inflected in a characteristic way to assume the adult configuration due to the cellular activities. In chick, the tubular heart be­comes ‘S’-shaped at the end of 3rd day after incubation. The heart becomes con­stricted in some regions and dilated at others.

In the 4th day of incubation, the atrial area expands into two lobes—the beginning of the left and right atria. The descending part becomes thickened to form the ventricle. The later development of heart is the differential growth and sub­division into chambers. Fig. 5.35 shows the twisting and formation of different parts of heart in a chick embryo.

The changes undergone by the tubular heart to form adult heart are essentially:

(i) Constrictions to form chambers.

(ii) Differential growth and thickening of the myocardium resulting in the for­mation of thin-walled receiving parts and thick-walled forwarding parts.

(iii) Kinking of the chambers—possibly due to rapid growth within crowded quar­ters.

(iv) Formation of septa, valves, etc.

Functional Changes in Cardiogenesis:

The definitive function of the heart starts as the paired cardiac tubes fuse and the contraction begins as soon as the primitive ventricle is formed. So the heart is the organ which begins its function at an early stage of development. Contraction starts in the myocardium along the right margin of the posterior end of the ventricle.

Gradu­ally the contraction involves the whole ventricular wall which contracts synchro­nously, i.e. periods of contraction alternat­ing with periods of rest. Meanwhile the atria develop which also contract at a more rapid rate. The atria control the rate of contraction of the heart as a whole.

These contractions set the contained blood in motion. Eventually pacemaker or sinoauricular node develops which takes the controls of the contractility of the heart as a whole.

Developmental Organ # 4. Kidney:

The kidney of vertebrates essentially consists of an aggregation of uriniferous units called nephrons. The kidney develops from the ‘intermediate mesoderm’ which lies between the somite and the lateral plate mesoderm. The intermediate meso­derm becomes segmented and each seg­ment is called the nephrotome. The nephrotome is transformed into the nephrons which involves significant cellular events.

A nephrotome contains a coelomic space, called the nephrocoel which communicates into the adjacent splanchnocoel by the peritoneal funnel (Fig. 5.36).

The nephro­tome is converted into a nephron in the following ways:

(i) The nephrotome, prior to its trans­formation, is a strand of cells between the somite and lateral plate mesoderm.

(ii) A tubular outgrowth develops from the dorsolateral wall of nephrotome.

(iii) The principle tubule originates from the tubular outgrowth which communi­cates with the nephrocoel through nephros- tome, i.e. the cavity of tubule is actually an extension of the nephrocoel.

(iv) The median wall of the nephrotome invests a tuft of blood vessels (arterial capil­laries) to form the renal corpuscle.

(v) The actual mode of origin of renal corpuscle is controversial. It was believed that Bowman’s capsule is formed by a pro­cess of invagination of the glomerular mass into the wall of the nephrotome.

But the electron-microscopic studies of Kurz (1958) have established that the double- walled Bowman’s capsule is not formed as a result of invagination, but due to a cleft within a compact cellular mass. The inner layer becomes reflected over the glomeru­lus while the outer one forms- the capsular wall.

The basic pattern of the development of nephron becomes greatly modified in diff­erent vertebrates.

The deviation is due to:

(i) Typically hollow nephrotomes are not found in embryos of higher vertebrate forms, instead the tubules develop within a continuous nephrogenic cord without exhibiting segmental disposition,

(ii) All the nephrons do not differentiate at a time, rather, the nephrons appear in a sequential order from the anterior to the posterior end.

(iii) The structural organisation of the nephrons also shows gradual complexity progressively from the anterior to the posterior end.

Developmental Events of Nephrons in Vertebrates:

In primitive vertebrates, the distinction between the anterior and posterior neph­rons is not well marked but in amniotes (reptiles, birds and mammals) the deve­lopment of nephric system shows the mani­festation of three distinct entities which succeed each other during ontogenic deve­lopment.

The entities are: Pronephros, Mesonephros and Metanephros. The fishes and amphibians possess first pronephros which gives way to the mesonephros—the final kidney of an adult. In amniotes, be­sides these two units, a third entity, the metanephros arises as the definitive adult kidney. All the types of nephrons exhibit a striking similarity in their cellular trans­port mechanism and physiological perfor­mances.

The segmental origin of the pronephric tubules is the character­istic feature in nephric development. In amphibians, the pronephric tubules are developed from the nephrotomes beneath third and fourth somites in salamanders and second, third and fourth somites in frogs.

It is to be noted that the number of the pronephric tubules corresponds directly to the number of segments involved. In chick, the pronephros develops from nephrotomes between fifth to sixteen somites.

The pro­nephric tubules begin to form when the embryo attains 12 to 13 pairs of somites (40-45 hours of incubation). The tubules become well developed in 16-21 somites stage. At 35-somite stage and at about 65-70 hours of incubation, the pronephros undergoes degeneration.

The sequence of events of transformation of the nephro­tomes into the pronephric tubules is clear in lower vertebrates, but in higher tetra- pods the stages are not so clear.

When several pairs of pronephric tubules are developed, they open into the coelomic cavity proximally while the distal ends join the pronephric duct (Fig. 5.37). The neph­ric duct is called the pronephric duct which not only serves as the drainage channel for the pronephros, but becomes involved with the development of mesonephros.

Typical pronephros is a functional kidney in the larval stages of fishes and amphibians. But in the embryos of reptiles, birds and mammals, the pronephros deve­lops in the anterior nephrotomes and is not functional at any stage. In human embryo, about seven pairs of pronephric tubules develop which start degeneration imme­diately after the initiation of the nephric duct.

Independency in the Differentiation of Pronephric Tubules and Nephric Duct:

The nephric duct (pronephric duct) starts development from the mesodermal blocks situated more posterior to the seg­ment from which pronephric tubules begin to form. In amphibians, the nephric duct originates from the nephrogenous meso­derm behind that which provides the pronephric tubules.

The somite 5 usually marks the level of the nephric duct primordium in amphibians. O’Connor (1938) has applied vital stain to pronephric swell­ings below the third and fourth somites in Ambystoma. It was observed that the stain appeared only in the pronephric tubules.

When the stain was applied below the fifth and seventh somites, the stain became confined to the nephric duct. Holtfreter (1943) has bisected the embryo between the levels of fourth and fifth somites and has observed that in the hind piece, though devoid of pronephric tubules, the nephric duct still develops perfectly.

The above ex­perimental fact relates that the pronephric tubules and nephric duct are determined, independently of each other. Once the nephric duct starts development, both the pronephric tubules and nephric duct elon­gate at a rapid rate. The pronephric tubules become thrown into loops as a result of elongation and the glomeruli of several segments may join together to form the glomus (Fig. 5.38).

The nephric duct (now designated as pronephric duct) after inauguration, pushes itself backward along the lower ends of the somites and the pos­terior movement is stopped as it reaches the cloaca. The duct fuses with the wall of the cloaca and its lumen opens into the cloacal cavity.

The backward elongation of the prone­phric duct towards the cloaca is possibly due to either by (i) progressive addition of new material or (ii) due to free terminal growth. Extensive literature exists on this particular issue. Overton (1959) advocated that the duct increases by independent caudal growth.

Holtfreter opined that the growth of pronephric duct towards the cloaca is due to selective cell-adhesions rather than chemotaxis as advanced by many. Holtfreter also suggested the role of blood vessel during the process, but this issue remains open for further investigation.

The mesonephros is deri­ved from the nephrotomes posterior to the pronephros. In majority of amphibians and amniotes, the component mesodermal cells of the nephrotomes dissolve into an aggregation of mesenchyme. These aggre­gated cells stretch on each side of the body along the dorsal margin of the lateral plates.

This mass of mesenchymal cells is called the nephrogenic cord or nephrogenous tissue. The mesonephric tubules develop from the nephrogenic cord extending bet­ween 17 to 30 somites in chick embryo.

The proliferation of mesenchymal cells leads to the formation of elongated solid cords. Each suchycord elongates and assu­mes ‘S’-shaped appearance. It becomes hollow to form a cavity. One end of suph a tubule connects itself to the existing prone­phric duct (the mesonephric tubules do not form a duct of their own), while the proximal end forms a double-walled Bowman’s capsule.

The pronephric duct is now designated as the mesonephric or Wolf­fian duct because it serves as a drainage duct for the mesonephros. The Bowman’s capsule is supplied by small blood vessels from the dorsal aorta (Fig. 5.39).

Several mesoephric tubules are developed in a segment, i.e. the number of mesonephric tubules do not correspond to the number of somites involved in nephrogenesis. When first formed, one mesonephric tubule deve­lops in a segment, but subsequently each tubule gives origin to secondary and ter­tiary tubules by budding (Fig. 5.40).

In case of chick embryo, the mesonephros becomes functional from 5th to 11th days. The tubular system becomes extensively coiled in 8th to 10th days of incubation. After this period the mesonephric tubules start degeneration along the anteropos­terior direction and their function is taken over by metanephros which differentiates subsequently in the region posterior to that of mesonephros.

Role of Pronephric Duct in Mesonephros Differentiation:

The mesonephrogenic cord will start differentiation into mesonephric tubules as soon as the pronephric duct is in touch with it. The mesonephrogenic cord develops mesonephric tubules only if stimulated by the pronephric duct.

From this observation it is natural to think that the pronephric duct serves as an inductor for the differentiation of the mesonephros. Extensive experimentations have been done on this issue to ascertain the induc­tive role of pronephric duct in mesone­phros differentiation.

Humphrey (1928), Burns (1938) and Holtfreter (1944) have experimentally obstructed the backward extension of the primordial nephric duct and have found the formation of mere clump of cells in the mesonephrogenic cord. Waddington (1938) and O’Connor (1939) have shown that the mesonephrogenic cord fails to develop renal tubules if the pronephric duct does not reach the specified region.

Local condensation of cells occurs only in the nephrogenic cord. Boyden (1927) by des­troying the tip of pronephric duct by cautery and Waddington (1938) by inci­sion of the duct have shown that the differentiation of mesonephrogenic tissue into the mesonephric tubules occurs only when the pronephric duct makes contact with the tissue.

But Gruenwald (1942) and Calame (1962) have cast doubt on such induction and reported that the mesone­phrogenic cord is capable of a consider­able degree of self-differentiation. Gruen­wald (1942) and van Geertruyden (1946) have shown that the nervous tissue, when transplanted into the competent mesone­phrogenic cells, can induce mesonephros differentiation.

So it is not unreasonable to think that other tissues (possibly somi­tes) are also involved in this process. So the generalisation that differentiation of mesonephric tubules depends solely upon induction by the pronephric duct appears premature to accept.

The metanephros is the functional kidney in the postembryonic life of amniotes. It develops from the neph­rogenic cord which is derived from the nephrotomes posterior to the mesoriephros adjacent to the cloaca. In chick embryo, the metanephros begins its deve­lopment at the end of the 4th day of incu­bation and between the 31-33 somites.

In the embryos of amniotes, the ureteric bud emerges as a diverticulum from the mesonephric duct near its junction with cloaca. This bud develops into the meta­nephrogenic cord or blastema. The distal end of the bud expands to form the pri­mordial renal pelvis. The metanephrogenic blastema starts condensation around the pelvis (Fig. 5.41).

The pelvis produces subdiverticula, each of which becomes the collecting tubule. The nephrogenic tissue accumulates around the distal end of each collecting tubule and forms ‘S’-shaped metanephric tubule (Fig. 5.42).

Each metanephric tubule opens into the collect­ing tubule at one end and the other end forms a double-walled Bowman’s capsule. The metanephros uses the mesonephric duct for the elimination of urine, but the
connection between them is not direct, and is established by means of a special outgrowth, the ureteric bud which trans­forms into the renal pelvis and the ureter.

Role of Ureteric Bud in Metanephros Diff­erentiation:

The conversion of the metanephrogenic tissue into metanephric tubules is dependent on an induction from the ure­teric bud developing from the mesonephric duct. Because the extirpation of either mesonephric duct or ureteric bud causes the failure, of the formation of metane­phros. This phenomenon suggests the inductive phenomenon.

It has been experimentally tested that the metanephric primordium undergoes characteristic development when cultured in vitro. The pelvis component develops a system of collecting tubules while the blastema forms coiled tubules. When these two components, after separation with trypsin, are cultured independently, nei­ther of them is able to carry through characteristic morphogenesis.

The subdivi­sion of the renal pelvis is dependent upon the metanephrogenic blastema, while the tubule differentiation in the blastema rests upon an inductive stimulus from the ure­teric bud. So the existence of inductive role’ played by ureteric bud in metane­phric differentiation seems to be positive.

Limitations and future directions

For many years, studies of cnidarian development, particularly of pattern formation, stem cells and regeneration, were dominated by research using the adult Hydra polyp, allowing only indirect comparisons with bilaterian embryonic development. However, molecular studies of cnidarian embryos and larvae have gained momentum with the introduction of Nematostella and Clytia as models. We expect that the recent technical advances in these systems will fuel research to better understand axis and germ layer evolution and to understand the origin of stem cells and neurogenesis. We also expect that, as more cnidarians are developed as models, particularly from taxonomic groups that have been little studied to date, such as scyphozoans, cubozoans and scleractinian corals, we will begin to understand the molecular basis for the dramatic morphological variation that exists among cnidarian lineages. Comparisons between these morphologically diverse species might provide insights into the constraints of the underlying developmental programs. Even though most cnidarians will never become laboratory models, their genomes hold important information regarding the evolution of developmental pathways in bilaterians. Thus, a goal for the future is the generation of draft genome sequences for more cnidarian species, followed by comparative analyses to identify conserved and diverged features of their gene sets.

Although the recent development of methods for genetically manipulating cnidarians has facilitated studies of gene function, many of the tools available for more mature model organisms are still lacking for cnidarians. A more robust RNAi approach and improved transgenic methods (e.g. with inducible promoters or landing sites for recombination) are needed. The identification of more cell- or tissue-specific promoters to drive the expression of fluorescent protein genes will enable morphogenetic processes, such as gastrulation and nervous system restructuring during regeneration, to be followed in vivo by four-dimensional confocal microscopy.

It remains to be seen whether classical genetic screens can be performed in cnidarians. Early attempts at this in Hydra were hampered by the low numbers of embryos that can be obtained and by the lengthy embryonic dormancy that most Hydra strains undergo. None of the responsible genes has been identified for the 39 existing Hydra mutants (Sugiyama and Fujisawa, 1978). Ongoing inbreeding programs with defined Nematostella strains might make mutant screens feasible in the future, but Nematostella has a relatively long generation time of 4-6 months. In this respect, the hydrozoan Clytia, which has a generation time of 3-4 weeks, perhaps holds more promise. Genetic mapping and cloning of the histocompatibility complex has been carried out in the hydrozoan Hydractinia (Nicotra et al., 2009), indicating that genetic approaches are feasible in cnidarians.

Although studies of cnidarian development to date have focused on axis formation and regeneration, cnidarian models offer exciting opportunities for investigating other aspects of development. For example, how development of the cnidarian nervous system is controlled and how its development relates to nervous system development in bilaterians are of obvious interest. Future efforts will certainly expand on recent studies using cnidarians to understand eye evolution, as cnidarians are the only animals among the four basal metazoan phyla that have evolved sophisticated eyes. Whereas many hydrozoan medusa have simple eyes at the rim of the bell, the most sophisticated eyes are found in the cubozoan jellyfish. Box jellyfish have a total of 24 eyes arranged in a set of four rhopalia, each consisting of two lens eyes and four bilaterally paired pigment cup eyes (Kozmik et al., 2008 O'Connor et al., 2009). Cnidarians have multiple ciliary opsins, the light-sensitive receptors found in photoreceptor cells (Suga et al., 2008). Furthermore, although they do not possess a bona-fide Pax6 gene, which is the so-called master regulator of eye development in bilaterians (Gehring and Ikeo, 1999), they have a related, putative ancestral PaxB gene that is likely to function in eye development (Kozmik et al., 2003 Nilsson, 2009 Nilsson et al., 2005 Suga et al., 2010). As the molecular circuitry underlying eye formation in cnidarians is defined in more detail, the degree to which eye development in cnidarians and bilaterians is evolutionarily related should finally be revealed. Since even eyeless cnidarians respond to light, the identification of the light-sensitive cells in such cnidarians should yield insights into the evolutionary origins of light-sensing organs in general.

In summary, with the availability of genome sequences and the advent of gene knockdown techniques and transgenics it is now possible to carry out experimental studies of developmental processes in cnidarians that would have been impossible only a few years ago. We look forward to finding the answers to long-standing questions, gaining new insights and revealing surprising findings from the continued study of these remarkable animals.

Watch the video: Basic pattern of early embryonic development in animals (December 2022).