Types of
egg Or Classification of egg:
There is
great variation in the shape, size and yolk content of egg in different animal.
The size of egg increases because of more and more deposition of reserve food
yolk.
The egg of Reptile and Birds are largest egg as compare to egg of other animals. The different types of egg are classified according to distribution of yolk and size of yolk granules and amount of yolk content.
Classification
Based On Distribution of yolk:
1. Alecithal
Egg:
The egg which lacks the yolk material
is called ‘’Alecithal egg’’. The size of Alecithal egg is small generally
this type eggs are of Viviparous mammals.
Example: Mammalian egg.
2. Homolecithal
Egg / Isolecithal egg:
If the yolk granules are evenly distributed
in the Cytoplasm such egg is called ‘’Homolecitha’’l or Isolecithal egg.
Example: Cerebratulus sea- urchin.
3.Centrolecithal
egg:
The egg is in which the yolk is concentrated in the
center. The cytoplasm is distributed as a thin coat on the external surface is
called ‘Centrolecithal egg’. In the unfertilized egg the nucleus is situated in
the center of yolk, Enclose in island of cytoplasm.
Example: Insect
egg- This egg contain moderate amount of yolk. There for is also called’’
mesolecithal egg’’.
4.Telolecithal
egg:
The egg in which when the yolk distribution is unequal, greatest concentration of yolk is at the vegetable pole of the egg. While animal pole (upper part) has no yolk, but relatively more cytoplasm at the animal pole such type of egg is known as ‘’Telolecithal egg’’.
Example: Frog egg and chick egg.
*Classification
based on size of yolk granules:
The yolk granules deposited in the egg are different sizes. Some yolk granules are small fine while in some egg. The protein yolk is found in the form of rather large granules called ‘’Yolk platelets.’’ The yolk platelets have a oval shape and are flattened in one plane and the cytoplasm is densely packed with in then depending on the size of yolk granules there are 3;type
1. Microlecithal egg
If the yolk
granules are small, microscopic, and very little amount of yolk is evenly
distributed of cytoplasm. Such egg is known as’’ Microlecithal egg.’’
Example: Echinoderm egg, Amphioxus egg.
2. Megalecithal egg
If yolk
granules are large, forming oval or flattened platelets large amount of yolk is
stored in the egg then it is called ‘’Megalecithal egg’’ Because of large
amount of heavy weight yolk is mostly concentrated at vegetal pole.
Example: Reptile
and bird egg.
3. Mesolecithal egg
The
egg contain good deal of yolk granules are of fairly large size, such egg are
called ‘’Mesolecithal egg.’’
Example: Amphibian egg.
*Egg
Membrane:
The mature
egg in all organism is covered by Plasma Membrane. Which is unique membrane,
made up of Protein and Lipid Belayed.
The thickness is about 50A. In addition to this membrane most of animal egg
show extra or Accessory protective layer of egg. Which are classified into
primary, secondary, tertiary egg membrane.
1. Primary egg membrane
It is secreted by itself and
it form next to the plasma membrane. Primary membrane are varied in different
egg of animal
a. Vitelline membrane
It is a thin transparent around the
plasma membrane and in parent in the insect, mollusk, Amphibians and Birds egg.
b.
Zona radiate
It is striated appearance next to the plasma membrane. It
found in egg of sharks some bony fishes and some amphibians and reptile.
c.
Zonapellucida
It is modified layer of zone radiate occur in
Mammalian egg.
d. Jelly envelop
It is like
jelly covering next to the plasma membrane. It occurs in echinoderm
egg and most
marine vertebrates egg. This marine membrane are structurally must thin.
2.Secondary egg membrane
This is secreted by ‘’Ovarium tissue’’ around the primary egg
membrane. These are composed of thick jelly coat in amphibians. It is called
chorion or chitineous shell in insect, Ascidians and cyclostomes.
3.Tertiory egg membrane
Theses
membrane are formed by Oviduct of the female reproductive system. These are
composed of Albumen Homycapsule or membranes in fish’s eggs. It is the form of
Albumen, form of shell membrane and hard calcareous shell around the plasma
membrane is in most Reptile and Bird egg.
*Significance of egg membrane:
1. In the egg of oviparous animal the egg membrane
provide protection to the content of egg
and also to developing embryo. In fact membrane protect the egg form Ecological
Izard (temp, radiation, pollution, desiccation).
2. It also help for mechanical injuries.
3.In viviparous animal egg membrane are important to avoid
egg fusion, to prevent polyspermy and they are necessary means to normal
cleavage after fertilization.