Question

Define the term cleavage. Name and describe the types of eggs on the basis of presence of yolk.

Answer 1

Cleavage means cell division, especially of a fertilized egg cell.

On the basis of amount of yolk eggs are 4 types -

(i) Alecithal - The yolk is almost absent. eg. Eutherian eggs

(ii) Microlecithal - They have very little amount of yolk. Romer and Balenski proposed the term oligolecithal. eg. Amphioxis and Tunicates.

(iii) Mesolecithal - They have moderate amount of yolk. eg. Amphibian, Dipnoi Fishes, Petramizon and Annelids worm.