Types of eggs

useful for undergraduate life science students

1. Types of Eggs

2. Based on amount of Yolk
•Alecithal
•When the yolk particles are
completely lacking
•Eutherianmammals
•MicrolecithalEggs:
•They contain very small
amount of yolk, e.g. eggs of
Sea urchin, Herdmania,
amphioxus.

4. •Mesolecithal Eggs:
•They contain moderate amount of yolk, e.g.,
eggs of lamprey, lung fish, frogs and toads.
•Macrolecithal (Megalecithal or Polylecithal)
Eggs:
•They contain large amount of yolk, e.g., eggs
of sharks, bony fishes, reptiles, birds and egg
laying mammals.

5. Eggs Based on Distribution of Yolk in
Cytoplasm
•Homolecithal Eggs:
•Yolk is uniformly distributed, e.g. eggs of
annelids, molluscs, echinoderms and
protochordates.

6. •Telolecithal Eggs:
•Yolk distribution is uneven
•Yolk is concentrated in the
vegetal half, due to gravity.
•The active cytoplasm is
present in the animal pole
•e.g. eggs of amphibians.

7. •Centrolecithal Eggs:
•Cytoplasm is distributed as
a thin layer at the periphery.
An island of cytoplasm is
also present in the center.
The nucleus lies at the
geometric centre of the yolk
mass, surrounded by a small
amount of cytoplasm.
•Yolk is localized between
the two layers of cytoplasm,
e.g. eggs of insects.

8. •Extremely Telolecithal
•Yolk is very large which
occupies nearly the entire
ooplasm, leaving free only a
small disc like area of
cytoplasm for the nucleus,
e.g., eggs of reptiles, birds
and egg laying mammals.
•In case of birds the egg is a
complete closed system
covered by shell. All the
essential material is present
inside the shell. Such egg is
also called as cledoic egg.