Egg membranes and their types in animals

Egg Membranes

The eggs are well protected by egg  membranes' The membranes are produced either by the egg itself or by the follicle cells of the ovary or by the genital ducts (oviduct) of the female, mother.Accordingly, the egg membranes are classified into three types. They

are .1. primary membranes 2. Secondary membranes and 3. Tertiary membrane.

III. primary membranes :The membranes secreted by egg cytoplasm (ooplasm) constitute the primary membrane. They are closely  attached to the surface of the egg. The primary membranes are named differently in the different animals. They are

a. Plasma Membrane

It is the membrane covering the egg immediately It over it. is found in all the.eggs.

in structure, It resembles the plasma membrane of a cell.

b.Vitelline Membrane: it is closely attached to the plasma membrane of egg. Commonly found in  Egg of Amphioxus. molluscs. Echinoderm s, amphibians birds etc It is very thin and and transparent. It is formed of mucopolysacharide and fibrous protein. The space formed between it and the plasma membrane is called perivitelline space filled with a  a

fl uid called perivitelline fluid.

c. Chorion:It is found in the eggs of lower chordates like fishes (styela)It is is a product of surface ooplasm

d.Zona Radiata

Th.e egg of the shark Scyllium canicula, has two primary membranei produced by the surface ooplasm. The outer membrane is the vitetline mbmbrane andthe inner membrane has

a radiating appeafance and hence called zona radinta The eggs-. of teleost fishes are also covered by zona radiate.

e.Zona Pellucida

All mammalian eggs are surrounded by a membrane called zona pellucida is also named as zone radiata.It is so named  because it gives a striated appearance under the microscope'

The striations are due to the presence of microvilli and macrovilli (desmosomes) in this zone. The microvilli are produced  by the surface of the egg and microvilli are produced by

Follicle  cells. They protrude into the zona pellucida

II. Secondary Membranes

The secondary membranes are produced by the follicle cells (cells found around the developing oocytes) ofthe ovary. These membranes are usually tough and jmpermeable' The secondary membranes are as follows

a. Chorion

This is a common outer covering in the eggs of insects, ascidians and cyclostomes (Myxine).It is found outside the vitelline membrane. As the chorion is tough and impermeable

it is provided with one or more openings called micropyles through which the sperms enter the egg

b. Corona Radiata It is found in mammalian eggs. This membrane is formed of a layer of follicle cells. The cells are radially arranged around the zona pellucida

III.Tertiary Membranes

The tertiary membranes are produced by the oviduct.

a. White Albumen

It is found in the egg of hen. It is found outside the vitelline membrane. It is formed of three layers-an inner less dense albumen, a middle dense albumen and an outer less dense albumen. The albumen is formed of water  and protein

b. Shell Membrane:The shell membrane is formed around the albumen in the egg of hen. It is a double membrane. The two membranes adhere closely and are separated by an air space ar the blunt end of the egg. This membrane is formed of keratin-

c. Shell:The shell is the outer covering of land animais eggs. It is formed of calcium carbonate. Lt is white or brown in colour.

It contains as many as 7000 minute pores. These pores are 0.04 to 0.05 mm in diameter. They are filled with  a proteinous substance called collagen.

d. Jelly Coat ,:The amphibian eggs are surrounded by a gelatinous covering called jelly coat

e. Mermaid's Purse

It is the egg case of some cartilaginous fishes. It is a protective hard shell secreted by the shell glands present in the oviduct. The shape of the purse varies from group to group.

Generally ,it is rectangular in shape. The corners of the shell are drawn out into four long twisted elastic filaments which serve to attach the eggs to sea weeds. In dog-fish Chiloscyllium,

development is completed within this purse