Due to presence of high amount of yolk, the macrolecithal eggs divide by meroblastic cleavage in which the cleavage furrow can not cut thought the heavy yolk completely. On the basis of yolk amount, eggs can be, alecithal (yolk absent, e.g., human), microlecirthal (e.g., tunicates, urchin), mesolecithal (lung fishes and toad), and amphibian macrolecithal (large amount of yolk, e.g., birds, reptiles, bony fish and prototherian mammals).