This document describes different types of eggs based on their yolk content and distribution. It identifies four main categories of eggs: alecithal eggs which lack yolk; microlecithal, mesolecithal, and macrolecithal eggs which contain small, moderate, and large amounts of yolk respectively. It also describes four patterns of yolk distribution: homolecithal where yolk is uniform; telolecithal where yolk is concentrated on one side; centrolecithal where yolk is localized between two cytoplasm layers; and extremely telolecithal where nearly all the egg is yolk except a small disc of cytoplasm. Examples are provided for each category.

1. Types of Eggs

2. Based on amount of Yolk
• Alecithal
• When the yolk particles are
completely lacking
• Eutherian mammals
• Microlecithal Eggs:
• 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.