Types of Cleavage: Type of cleavage in an egg is de termined by amount and distribution of yolk.
1. Holoblastic cleavage: Cleavage of zygote is com plete and divide 2 blastomores completely. This is of
2 types:
(i) Equal holoblastic cleavage:
• It forms equal blastomeres. e.g : Human, starfish
• Alecithal and microlecithal egg.
(ii) Unequal holoblastic cleavage: It forms 2 unequal blastomeres. e.g : Frog, dog fis: Mesolecithal egg.
2. Meroblastic cleavage: In this cleavage is incomplete or partial or parietal and the division is confined to the animal pole. Yolk is not divided. It is of two types
(i) Discoidal cleavage: The cleavage confined only to the disc area at the animal pole, e.g.: Reptiles, birds (chick and pigeon) and egg laying mammals : Megalecithal egg.
(ii) Superficial cleavage: The cleavage is restricted to the superficial region of egg. e.g : Insects (Arthro pods), Centrolecithal eggs