Embryology(Part-2) - Notes, Zoology, Class 12 PDF Download

EMBRYOLOGY

TYPES  OF  EGGS

(A) On the basis of amount of yolk 

(i) Alecithal - In this type of egg, yolk is negligible. Ex. Human egg.

(ii) Microlecithal or Oligolecithal eggs -- The amount of yolk is very small in these types of eggs. (oligolecithal , or microlecithal or alecithal).

Examples :- Egg of Amphioxus, Eutheria, Metatheria and sea - urchin.

(iii) Mesolecithal Eggs :- In this type of egg, the amount of yolk is moderate i.e. medium, neither more nor less.

Example - Eggs of Amphibia , Petromyzon and lung-fishes.

(iv) Polylecithal or Macrolecithal or Megalecithal eggs :- Eggs are with large amount of yolk. e.g.

Insect's egg, Birds, reptiles and proteotheriam mammals

(B) On the basis of destribution of yolk : 

(i) Isolecithal or homolecithal eggs : The yolk is evenly or homogenously distributed in these eggs. eg. : micro, oligo or alecithal eggs.

(ii) Telolecithal eggs : The yolk is concentrated in one part of the egg. eg. : mesolecithal eggs of amphibia. (Moderately telolecithal) Discoidal eggs A type of teloleithal and megalecithal eggs, Where the yolk is in enormous quantity and concentrated in one part of the egg. Thus only a disc of cytoplam called germinal disc remains in the egg which is located at the other pole of egg. (Heavily telolecithal) eg. : Eggs of reptiles, birds and prototherian mammals.

(iii) Centrolecithal eggs : Megalecithal eggs where the enormous amount of yolk is located in the centre and cytoplasm is in the form of superficial layer around the yolk. eg. : Insects egg.

(C) Classification of Eggs on the basis of Shell :-

On the basis of shell, eggs are of 2 types :-

(i) Cleidoic eggs :- eggs surrounded by a hard shell are known as cleidoic eggs. These eggs are found in those animals which have a terrestrial mode of life or which lay eggs on land.

These eggs have more amount of yolk. These are adaptations to terrestrial mode of life. Shell prevents the egg from dessication. e.g. :- eggs of "Reptiles", "Birds", "Terrestrial Insects " and "Prototherians".

Reptilia eggs are called leathery eggs.

(ii) Non - Cleidoic eggs :- Eggs which are not surrounded by a hard shell are called non-cleidoic eggs. eg. :- all viviparous animals (Mammals) and all oviparous animals which lay eggs in water (Amphibians).

STRUCTURE OF AN OOCYTE

The nuclues of egg is also called germinal vesicle.

Oocyte is surrounded by membranes termed as the egg-membranes.

Oocyte / Ovum along with the egg-membrane are termed as the egg.

Egg = Ovum / Oocyte + Egg membrane.

Majority eggs are oval but the eggs of insects are long and cylindrical. Smallest eggs are of 50m in polychaeta and the largest eggs are of an Ostrich.

Classification of egg - membranes :-On the basis of origin, egg- membranes are of 3 types :-

(1) Primary egg membrane :This membrane is secreted by the oocyte  itself. eg. Vitelline membrane, Zona Pellucida (mammals), Zona radiata (Shark and Some amphibians)

(2) Secondary egg membrane : This is found outside the primary egg membrane and is secreted by the ovary. (eg. Corona radiata, Chorion)

(3) Tertiary egg membrane : This is present outside the primary egg membrane. It is either secreted by the uterus or the oviduct. (eg. Jelly coat, Shell & Shell membrane)

Functions of Egg-membranes 

(i) To provide protection

(ii) To check polyspermy

(iii) To provide buoyancy to the amphibian eggs

Different types of eggs

(I) INSECT EGG

Eggs of insects are megalecithal or polylecithal In them yolk is present in the centre, so the eggs are also centrolecithal.

Two egg membranes are present here, inner vitelline membrane (primary) and outer chorion (secondary).

The sperm enters the egg through micropyle because on the head of insect sperm acrosome is absent.

The Cytoplasm here is found in two parts :

(a) Central cytoplasm :-It is present in a very small amount in the center of the egg. Egg nucleus is located in it.

(b) Peripheral Cytoplasm :- It is present in a very small amount along the periphery of the egg.

(II) FROG'S EGG

Eggs of frog are moderately Telolecithal & Mesolecithal .

Two types of egg membranes are found in frogs egg :

(i) Vitelline membrane : This is primary egg membrane which is secreted by the ovum around itself.

(ii) Jelly coat : This is tertiary egg membrane, secreted by oviduct. Jelly coat has air bubbles trapped in it due to which it floats an water. This group of frogs egg is called spawn, Jelly coat is bitter in taste so enemies do not eat it.

Secondary egg membrane is absent in frog's egg.

Internal part of the egg is divided in two parts : 

(i) Animal pole : This part has cytoplasm, egg nucleus in also located in this part. In the cytoplasm melanin granules are found which prevent the egg from harmfull radiations. They also help in protection of egg by camouflage.

A sperm always enters into the ovum at some point in animal hemisphere. This point is normally other than the animal pole itself.

As the sperm enters into the ovum, taking some pigment granules with it, a grey, crescent shaped region appears in the equatorial zone geometrically opposite to the sperm entrance point. This region is called grey crescent. It is formed due to movement of some pigment granules away from it towards sperm entrance point.

The area of sperm entrance point marks the anterior side of future embryo. The side diagonally opposite to it in the vegetal hemisphere marks the future posterior side. Thus the sperm entrance establishes the anteroposterior and dorsoventral axis as well as bilateral symmetry of future embryo.

(ii) Vegetal pole : yolk is concentrated in this part of egg.

(III) CHICK EGG

These eggs are megalecithal or polylecithal and discoidal eggs.

In these eggs, yolk is present in the centre of the egg in the form of a dense mass. The cytoplasm of the egg is in the form of a disc above the yolk, which is termed as the germinal- disc.

Yolk is of 2 types i.e. yellow yolk and white yolk.

Yellow - yolk has more amount of phospholipids. White yolk has less amount of phospholipids. Central white part of yolk is called latebra.

Both the types of yolk are arranged in  alternate and concentric layers.

Pander was the scientist , who discovered the 3 germinal layers i.e. Ectoderm Mesoderm and Endoderm in chick - egg.

Around the egg, a porous shell of CaCO3 is present which is secreted by the cells of posterio