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Essay on Embryology

Essay Contents:

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.

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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.

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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.

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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.

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.

Yolk:

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.

Types of 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.

Spermatozoa:

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.

1. Head:

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).

2. Middle Piece:

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.

3. Tail:

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.

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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 fertilisation,

B. Internal fertilisation.

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.

Cleavage:

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.

Types of Cleavage:

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.

Pattern of Cleavage:

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.

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Essay # 8. Stages of Embryogeny:

1. Morula:

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.

2. Blastula:

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.

vi. Blastocyst:

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.

3. Gastrula:

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.

4. Organogeny:

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:

i. Ectoderm:

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.

ii. Endoderm:

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.

iii. Mesoderm:

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.

(i) Epimere:

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.