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The following points highlight the two types of eggs. The types are: 1. Eggs Based on Quantity of Yolk 2. Eggs Based on Distribution of Yolk in Cytoplasm.
Type # 1. Eggs Based on Quantity of Yolk:
1. Microlecithal Eggs:
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They contain very small amount of yolk, e.g. eggs of Sea urchin, Herdmania, amphioxus. The eggs of man contain very little amount of yolk, hence human egg is alecithal (almost free of yolk).
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2. Mesolecithal Eggs:
They contain moderate amount of yolk, e.g., eggs of lamprey, lung fish, frogs and toads.
3. Macrolecithal (Megalecithal or Polylecithal) Eggs:
They contain large amount of yolk, e.g., eggs of insects, sharks, bony fishes, reptiles, birds and egg laying mammals.
Type # 2. Eggs Based on Distribution of Yolk in Cytoplasm:
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1. Homolecithal Eggs:
Yolk is uniformly distributed, e.g. eggs of annelids, molluscs, echinoderms and protochordates.
2. Telolecithal Eggs:
Yolk is concentrated in the vegetal half, e.g. eggs of amphibians.
3. Meiolecithal Eggs:
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.
4. Centrolecithal Eggs:
Yolk is localized at the centre, e.g. eggs of insects.
