3. Structure of eggs:
The fully developed female gamete is called as ‘ovum’ or ‘egg’.
It is non motile. The eggs are relatively larger than the male gametes. The egg
performs different functions like, it provides haploid set of chromosome
to the future embryo, and also it provides cytoplasm and food to the embryo.
In egg, contains yolk and albumin. The yolk is yellowish colour and albumin
is whitish colour in nature. Yolk is present inside the albumin. The egg is sperical or
Ovoid In shape. But in a few animal like insects, the eggs are elongated and cylindrical
In nature.
4. Types of eggs:-
On the basis of amount and distribution of yolk in the eggs, the eggs are
classified by the embryologists as follows:
A. Types of eggs based on the amount of yolk:
On the basis of amount of yolk present in the eggs, the eggs are classified
into four types:
1.Alecithal eggs
2.Microlecithal eggs
3.Mesolecithal eggs
4. Macrolecithal eggs
1.Alecithal eggs:- The ovum consist of almost no yolk in it. Animal like eutheian mammals
produce such eggs.
Nucleus
2.Microlecithal eggs:-
The eggs which contain small amount of yolk or reserved food, such eggs are called
“Microlecithal eggs”.
Such eggs are found in amphioxus, tunicates, cephalocordates .
5. 3.Mesolecithal eggs:-
Mesolecithal eggs have comparatively more yolk
than the microlecithal eggs.
the eggs which contains moderate amount of yolk,
such eggs are called mesolecithal eggs.
The mesolecithal eggs are found in petromyzon,
dipnoi and amphibia.
4.Macrolecithal eggs:-
Eggs with a enormous (large) amount of yolk are called
macrolecithal .Macrolecithal egg are found in fish, reptile, birds .
Macrolecithal eggs also called Megalecithal or Polylecithal eggs.
6. .
B. Types of eggs based on the distribution of yolk:
On the basis of distribution of yolk , the eggs are classified into three type:-
1. Homolecithal or Isolecithal eggs
2. Telolecithal eggs
3. Centrolecithal eggs
1.Homolecithal or Isolecithal eggs:-
Egg of this type have little amount of yolk disbursed uniformly in the entire
cytoplasm.
All microlecithal eggs have this nature. The microlecithal eggs are Homolecithal.
E.g:- Echinoderms, Amphioxus and Mammals.
2. Telolecithal eggs:-
Eggs containing large quantity of yolk. the eggs in which the yolk is not
distributed evenly. It is present at one side of the egg, may be at animal or vegetal pole. Such
eggs are called as ‘Telolecithal eggs’. The mesolecithal and macrolecithal eggs are telolecithal
eggs
.3.Centrolecithal eggs:-
The eggs in which the yolk is present
in the centre of the egg cytoplasm, such eggs are
known as ‘Centrolecithal eggs’. The eggs of
insects are centrolecithal eggs
7. Eggs of different animal groups:-
Fish and amphibian eggs:
The most common reproductive strategy for fish is
known as oviparity, in which the female lays undeveloped eggs
that are externally fertilized by a male. Typically large numbers
of eggs are laid at one time and the eggs are then left to
develop without parental care. The eggs of fish and
amphibians are jellylike. Fish eggs are Macro-Lecithal egg and
amphibian are Meso-Lecithal.
Bird eggs:
Birds eggs are laid by females and incubated for time that
varies according to the species; a single young hatches from
each egg. Average clutch sizes range from one to about 17.
The default colour of vertebrates eggs is the white of the
calcium carbonate from which the shells are made.
Most bird eggs have an oval shape, with one end rounded
and the other more pointed. This shape results from the egg
being forced through the oviduct
8. Amniote egg:
Like amphibians, amniotes are air – breathing vertebrates, but they have
complex eggs or embryos, including an amniotic membrane. Amniotes include
reptiles and mammals.
Reptile eggs are often rubbery and are always initially white. They are able
to survive in the air. Not all reptiles lay eggs; some are viviparous (“live birth”).
e.g. sea snakes, boas, skinks and slow worms
9. .
Reference
• Woods, Vanessa B., and Anna M. Fearon. "Dietary sources of
unsaturated fatty acids for animals and their transfer into
meat, milk and eggs: A review." Livestock Science 126.1-3
(2009): 1-20.
• Gautron, Joël, et al. "Production factors affecting the quality
of chicken table eggs and egg products in Europe." Animal 16
(2022): 100425.
• Rafferty, Anthony R., and Richard D. Reina. "Arrested
embryonic development: a review of strategies to delay
hatching in egg-laying reptiles." Proceedings of the Royal
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