27

27.1. INTRODUCTION

You have already learnt that life of all animals starts as a simple cell (zygote) whether laid as an egg outside the body of the female or retained inside the mother's womb, the zygote undergoes developmental changes, the developing embryo must be supplied with nourishment for growth. In this lesson, you will learn about the course of development and nutrition- of the bird and mammalian embryos.

27.2. OBJECTIVES

After completing this lesson you will be able to

list and define steps of gametogenesis, fertilization, cleavage, morula, blastula, gastrula, morphogenesis, organogenesis and embryogenesis.

describe spermatogenesis and oogenesis.

describe types of cleavage and blastula.

Define epiboly, emboly, invagination, delamination, ingression and involution with reference to gastrulation;

define embryonic nutrition, yolk, placenta, placentation and implantation;

state the formation and role of chorion, amnion, allantois and yolk sac during the development of chick and human foetus;

list the functions of placenta;

27.3. THE STEPS BETWEEN ZYGOTE AND ADULT - LISTED AND DEFINED

1. Gametogenesis: The zygote itself is a fusion product of sperm (male gamete) and egg (female gamete); the process by which these gametes are formed is called gametogenesis.

2. Fertilization: This consists of the fusion of the sperm, and the egg cell including their nuclei to produce the zygote.

3. Cleavage: The zygote undergoes repeated cell divisions (cleavages) to produce a large number of similar cells.

4. Blastula: The cells of morula arrange themselves so as to enclose a cavity (blastocoel) surrounded by a layer of cells.

5. Gastrula: Further rearrangement of cells converts the blastula into a three-layered structure with a new cavity (archenteron) and is called gastrula.

6. Morphogenesis and Organogenesis: The gastrula shows changes in shape and size. This external, change in form is Morphogenesis. Organogenesis is the developement of organs. All these steps from 1 to 6 are grouped together under embryogenesis. All stages from cleavage onward are called as the embryo.

27.4. MEANING OF DEVELOPMENT

The whole process leading to the formation of an adult from an egg is known as development. The developmental process consists of three features :

1. Growth,

2. Cell Movement,

3. Cell Differentiation

1. Growth: Growth is a more or less permanent increase in the amount of living matter or protoplasm.

2. Cell Movement: Tissues and organs occupy specific places in the body. To achieve these predetermined positions cells in the embryo have to move in a planned manner to their future places of occurence.

3. Cell Differentiation: Once the cells occupy specific positions due to the cell movement, they in turn differentiate themselves into the cells, tissues or organs to perform specialized function according to the place they occupy in the body of the organisms.

(a) Process of development require lot of energy. The embryo must therefore be continously supplied with food and oxygen.

(b) Throughout embryogenesis the non-living matter (food) is converted into more and more of living matter or protoplasm.

Developmental process in the unicellular organisms is much simpler because:

(a) They have to increase in cell size only and acquire the organelles.

(b) division of labour is restricted to organelles of the cell.

The same process in multicellular organisms is highly complicated because:

(a) There is tremendous increase in the number of cells.

(b) Division of labour requires tissues and organs from an initial cluster of similar cells, to a large variety of cells, performing different functions to be formed.

27.5. GAMETOGENESIS

It is the first phase in the sexual reproduction of animals. It consists of spermatogenesis (formation of sperm) and oogenesis (formation of ovum). The similarities between them are: . .

1. Both produce gametes that are haploid (n).

2. Both are specialized cells and do not resemble the original diploid (2n) cell.

3. They do not divide further.

The dissimilarities between them are shown in Table 27.1

Table 27.1. Differences between spermatogenesis and Oogenesis

Spermatogenesis	Oogenesis
1. Occurs in testes	1. Occurs in ovaries
2. One diploid cell gives rise to four haploid sperms, all are Functional.	2. Only one functional haploid ovum or egg is formed, the other three cells (called polar bodies) degenerate.(fig.27.1b)
3. Meiosis is completed within Testes and haploid cells Called spermatids are produced	3. Meiosis is prolonged. Completed after sperm entry (except in some Invertebrates)
4. Spermatids differentiate. - sperhriogenesis	4. No obvious differentiation, but egg cell increases in size.
5. Sperms of different animals look very similar. They all have a Middle piece and a tail. 6.All sperms are motile, adapted to swim, and produced in much larger number. 7. Sperms are much smaller than Egg cells.	5. Eggs differ greatly in their size, amount of the food contained and its distribution. 6, Eggs are non-motile, adapted to receive the sperm and start off the developmental process, and not produced in large numbers unlike sperm. 7. Eggs can be very large (birds eggs) but even the egg cell (mammals) is much larger than the sperm cell.

27.6. EGG FERTILIZATION AND DEVELOPMENT

Fertilization is the fusion of two gametes (sex cells) a male and a female one followed by the joining together of the nuclei of the two gametes Events in Fertilization are given below and shown in fig 27.2

Sperm contacts the egg membrane with its acrusome.

Many sperms may reach the egg but finally only one fuses with the egg.
The entry of sperm activates the egg for completing meiosis.
Tail of sperm is left outside the egg only head with nucleus enters along with middle piece.
The two haploid nuclei fuse to become a diploid nucleus and a

zygote is formed.

Fertilization may take place inside or outside the Mother's body. This in turn determines the types of development whether internal or external.

27.7. CLEAVAGE AND BLASTULA

Cleavage can be characterised as that period of developement in which :

1. The fertilized egg (unicellular) is transformed by consecutive mitotic divisions into a multicellular complex.

2. No growth occurs.

3. The general shape of embryo is not changed, but a cavity in the interior is formed called the blastocoel.

4. In addition to the transformation of cytoplasmic substance into nuclear substance, qualitative changes in the chemical composition of the eggs are limited.

5. The constituent parts of the cytoplasm of the egg are not much displaced and remain in general in the same position as in the egg at the beginning of cleavage.

1. Characteristics of Cleavage

(a) Cleavage occurs soon after fertilization.

(b) It occurs by repeated cell divisions and mitosis.

(d) There is rapid synthesis of nuclear matter (DNA, RNA) and increased consumption of oxygen.

(e) It ends with the formation of a blastula.

2. Characteristics of Blastula

(a) It is a ball of cells.

(b) The rate of cleavage slows down and a cavity is formed called blastocoel.

27.8. GASTRULA

The process of gastrulation is one of displacement of parts of the early embryo (fig.27.3). As a result the endodermal and mesodermal organ rudmients are removed form the surface of the embryo where the respective organs are found in the differentiated animals. At the same time the single layer of cells, the blastoderm gives rize to three germinal layers - the ectoderm, the endoderm and the mesoderm. It encloses a new cavity called archenteron.

(a) These three layers results due to definite movements of cells in and around the embryo.

(b) Gastrulation initiates morphogenesis and organpgenesis.

(c) The three layers known as the primary germinal layers are important as all the tissue and organs are derived from them

27.9 MORPHOGENESIS AND ORGANOGENESIS

The first external change that appears in the embryo is that it becomes elongated.

Next the head end and the tail end can be gradually distinguished, along with an upper and a lower side.

Finally the head lobe with position of eyes, mouth etc. and the beginning of fore and hind limbs can be gradually observed.

The following table 27.2 lists the organ:, or systems which develop from the germ layers in organogenesis.

TABLE 27.2 ORGANOGENESIS FROM GERM LAYERS

Germ. layer	Organs derived
Ectoderm	(i)	Epidermis
	(ii)	Nervous system '
	(iii)	Mouth and anal cavity
Mesoderm	(i)	Dennis
	(ii)	Muscle and bones
	(iii)	Connective tissues
	(iv)	Kidneys, gonads and heart
Endoderm	(i)	Alimentry canal and digestive glands.
	(ii)	Thyroid and pituitary glands
	(iii)	Lungs and gills.

27.10. EMBRYONIC NUTRITION AND ITS PRINCIPAL WAYS

Embryonic Nutrition:

The supply of nutritive food material to the developing embryo for the formation of new protoplasm and for embryonic nutrition.

The material required for development of the embryo as nutrients are a Carbohydrates, Lipids, amino acids, several minerals, water and oxygen.

The Methods of Embryonic Nutrition:

From food reserves contained in the egg.

From the mother during the development of embryo in the womb.

Egg of oviparous and ovoviviparous animals contain yolk which is utilized by the developing embryo. The yolk contains all the required nutrients. Examples: fishes, amphibians, reptiles, birds and primitive mammals.

In viviparous animals, (such as humans) the eggs have hardly any food reserve. In these the embryo receives nourishment from the mother through a special structure called placenta and remains enclosed in the womb (uterus) until birth (Fig. 27.4).

27.11. EMBRYONIC NUTRITION IN BIRDS

Birds are oviparous animals. They lay large yolky eggs. e.g hen's egg (fig.27.5).

At the time of laying, the hen's egg is covered by a porous calcareous (CaCO₃ ) shell. The shell is lined by a double shell membrane. At the blunt end of egg the two shell membranes are separated enclosing an air space.

Located in the centre of the egg is a large sphere of yolk, and a small quantity of living protoplasm which is enclosed by a thin vitelline membrane.

In between the shell membranes and the vitelline membrane lies the albumen or white of the egg.

The albumen is of two types: (i) thick dense albumen, and (ii) thin albumen.

The thick albumen forms a closed sac completely surrounding the yolk, and attached at the end of the egg to the shell membrane.

The thin albumen forms a layer between the thick albumen and the viteline membrane, and another layer between the thick albumen and the shell membrane

The inner layer of the thick albumen forms two twisted cords (chalazae) due to rotation of the ovum on both sides of the yolk.

Between the vitelline membrane and the yolk is present a protoplasmic disc (dorsally) consisting of cytoplasm and nucleus which later on develops into an embryo if fertilized.

The eggs are incubated by the warmth of the body of the mother (hen) who sits upon her eggy. A temperature of about 39° C is maintained and the chick hatches after 21 days.

Functions of Egg Parts

Shell is for protection and exchange of gases.

Albumen serves as food (a solution of glycoprotien in water) and also provides a wet environment to the developing embryo.

Yolk also serves as food.

Chalazae keeps the yolk in position so that germinal disc is not damaged.

The digestive enzyme secreted by the cells of the yolk convert the yolk into a soluble form, which is then taken to the heart by the vitelline veins and is then distributed to all parts of the developing embryo.

The yolk is not completely absorbed during embryonic life, shortly before hatching, the yolk sac is retracted into the abdominal cavity of the embryo, and the walls of the abdominal cavity close behind it.

The amnion is formed as a double layered structure over the embryo and enclosing it, the amniotic space between the embryo and amnion is filled with a fluid (amniotic fluid) which lubricates the surface of embryo as well as protects it from jerks (fig. 27.6).

27.12 EMBRYONIC NUTRITION IN MAMMALS

Mammals exhibit two types of embryonic nutrition, viz.

1. In lower mammals i.e. prototherians (egg laying mammals) the embryonic nutrition is derived from the food reserves in the egg like that in the birds.

2. In higher mammals including humans, the egg is devoid of yolk, the developing embryo gets the nutritive material solely from the mother.

The alecithal (yolkless) egg released by the ovary reaches fallopian tube (oviduct) where it is fertilized by the sperm.

Implantation

The blastocyst moves down the oviduct to reach the uterine wall, gets attached to uterine wall and starts developing. This is called implantation.

The time taken for implantation varies in different groups of mammals i.e. 6 days in rats, and 8 to 10 days in man.

Before implantation in the uterus the blastocyst is nourished by a secretion of the uterus ("uterine milk").

After implantation, an organ called placenta develops which connects the embryo (foetus) with the uterine wall.

Placenta is an organ consisting of embryonic and maternal tissues in

Close union, by which an embryo of a viviparous animals draws its

nourishment and oxygen from the mother.

Thus, the placenta establishes an organic connection between the foetus and the mother. The nutrients and Oxygen from the mother's blood are passed to the foetus and waste products are eliminated through it and then, to the outside via the mother's blood.

Functions of placenta (physiology of placenta)

It permits selective exchange of substances between developing embryo and mother by diffusion or by active transport. The chief functions of placenta are:

Nutrition: Transport amino acids lipids, glucose, fructose, vitamins, minerals, water and a small quantity of hormone from mother to the foetus.

Respiration: Oxygen from the maternal blood diffuses into the foetus and Carbon dioxide from foetus passes into the maternal blood stream.

Excretion : Nitrogenous wastes like urea, and uric acid produced as a result of metabolic activities in the embryo escape through the placenta into the maternal system.

Barrier : The placenta allows only the useful substances to pass into the foetus but prevents harmful ones including bacteria from passing though it. But viruses of small pox, German measles (rubella) and certain other diseases can-pass through it. Similarly, antibodies, antitoxins, anti-Rh agglutinins and certain drugs can easily pass through the placenta.

Storage : The placenta stores fat, glycogen and iron for the embryo before the formation of liver in it.

Secretion of Hormones : Placenta secretes certain hormones which help in maintaining pregnancy and prevent premature birth, and the hormone relaxin which helps in birth by stimulating the contraction of muscles of uterus. Placenta becomes more permeable in the later half of pregnancy than during the first half due to age and injury. Placenta has a fixed life span.

WHAT YOU HAVE LEARNT

The developmental steps and stages in sexually reproducing animals are gametogenesis,cleavage, blastula and gastrula, followed by morphogenesis and organogenesis.
In the entire process the three main features are growth, cell movement and cell differentiation.
Growth is required to produce a large number of cells, cell movement to make them occupy right places for differentiation and cell differentiation to make them specialized cells for definite functions.
Gametogenesis involves production of sperms and egg cells.
Fertilization leads to the formation of zygote, and may be internal or external.
Animals may be viviparous, oviparous and the ball of cells hollow out to form blastula.
Gastrula has the three germinal layers from which all tissues and organs are derived.
Internal organization of tissues and organs results in external changes in form by organogenesis and morphogenesis.
Embryonic nutrition is the supply of the required material for the synthesis of protoplasm and other compounds and for the production of energy during embryonic development.
Materials required are carbohydrates, lipids, amino acids, water and minerals.

Two principal ways of embryonic nutrition are :

From yolk present in egg and
From mother through the placenta during embryonic development in the womb.

In birds embryonic nutrition is from the yolk present inside the egg.
Hen's egg is made of

Shell for protection and, exchange of gases
Albumen for food and providing a wet environment.
Yolk for food and
Chalazal cords to keep the yolk in position.

Yolk contains protein, Phospholipid, glycogen and micro metabolites.
Placenta is a structure (organ) formed partly by the inner lining of uterus and partly by the extra embryonic membranes of foetus. The substances are exchanged through the placenta between the blood of mother and of embryo.
Umbilical cord-it is a vascular cord that serves as a communication between the foetus and its placenta.
Placenta serves for nutrition, excretion, mechanical barrier, storage of food and secretion of hormones.

TERMINAL QUESTIONS

1. Make a list of the steps of embryonic development.

2. What is the significance of cell differentiation ?

3. List the differences between spermatogenesis and oogenesis.

4. Enlist the main features of cleavage.

5. Name the tissues and organs that are derived from mesoderm.

6. List the nutrition requirements of an embryo.

7. What are the two principal ways by which the embryos obtain their nourishment ?

8. List the main functions of placenta.

CHECK YOUR ANSWERS

27.1

1. Blastula encloses blastocoel surrounded by a layer of cells whereas gastrula encloses archenteron surrounded by 3 germlayers

2. cleavage produces a large number of similar cells.

3. Embryo is a multicellular structure derived from the zygote. It later develops into a new individual through definite stages.

27.2

1. Cell division because lot of cells are required for differentiating into different types. Cell differentiation to make them specialized cells for definite functions.

2. from breakdown of food available to the embryo.

3. Growth is more or less permanent increase in the amount of living matter.

27.3

1. Egg and sperm are formed after meiotic division and are haploid(n). On fertilization they become diploid (2n) thus reconstituting the original number of chromosomes.

2. The haploid spermatid cells become a structure with head, middle paece and tail which is the sperm.

3. Four, one, diploid.

27.4

1. Fusion of egg and sperm 2. Viviparous : in man, cow.

3. Reptiles and birds. 4. For the purpose of cell differentiation

5. Blastula

27.5

1. A new cavity which is formed inside, the gastrula.

2. Germinal layers give rise to all the tissues and organs of the body.

3. Gastrulation

4. (i) Endoderm, (ii) Mesoderm (iii) mesoderm, (iv) Endoderm

27.6

1. (i) Embryonic nutrition (ii) Yolk, egg

2. Serves as food and provides wet environment for development of egg.

3. Digestive enzymes convert yolk into soluble form which is taken to the heart and then distributed to all parts of the embryo.

27.7

(i) Placenta

(ii) Implantation

2. An organ consisting of embryonic and maternal tissues in close union, by which embryo draw its nourishment and O₂

3. Nutrition, respiration, excretion, storage.