| Primary egg Membrane formed from oviduct. |
| Secondary formed from follicle. |
|---|
| Tertiary formed from oviduct or other secondary structure. |
Egg Membrane are of three types.
1. Primary Egg Membrane
2. Secondary Egg Membrane
3. Tertiary Egg Membrane
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The egg shell like all other cells, is covered by plasma membrane or plasmalemma. In addition to this , the eggs of most animals are surrounded by one or more accessory envelope called Egg Membranes. These are of three different types-
1. Primary Egg Membrane:
These are those Membranes which are formed from the ovum itself. The primary egg Membranes bears different names in different animals.
a) Vitelline Membrane
b)Zona Rediata
c)Zona Pelluceata
d)Jelly Envey
a) Vitelline Membrane :
It is thin and transperant membrane, closely applied to the surface of the under lying plasma membrane.
Eg. Egg Membranes in Insect, Molluscs, Amphibians, & Birds .
b) Zona Rediata:
it is so called because of it's striated appearance and has wide distribution among vertebrates eggs such as shark, certain bony fishes, Amphibians & Reptily.
c) Zona Pelluceata:
It is unstriated & takes to the place of zona Rediata of young mammalian oocyte.
d) Jelly envelope:
The eggs of sea urchin and many other Marine invertebrates are surrounded by a thick structure less jelly coat which belongs to the category of primary egg Membrane.
2. Secondary Egg Membrane:
These are produced by the follicle cells immediately surrounding the developing oocyte. This membranes are usually hard & impermeable. They are following types-
a) Chitinous Shell:
These surround the egg of insect ascidians & cyclostomes.
Eg. Petromyxon.
b) Chorion:
In insects and acidians the follicle cells secrets a second thicker membrane around the vitelline membrane called Chorion. It is tough and intermediate, produced with a single opening called micropyle at the anterior end.
C) Ovarian follicles in Mammals:
Although true secondary egg membrane are lacking the cells comparing the ovarian follicles, within which the egg lies are ovarian in nature. Thus theva (externa and interna) membrane granulosa and corona rediata are designated as secondary egg membranes in mammals.
3.Tertiary egg membrane:
Tertiary Egg Membrane are secreed by the oviduct or other accessory part of the female genital system. They are following types-
A) Albumen Coat:
The eggs of amphibians are surrounded by albuminus covering which protects the eggs and sometimes serves to make the eggs adhere to one another.
B) The mermaid purse:
In the oviparous sharks and Rays, the eggs is surrounded by a protective egg case called the mermaid purse. It is a hard shell secreted by the shell gonads present in the oviduct.