As mentioned, the amount of yolk in an egg and its distribution affect how cleavage occurs. In animals that have isolecithal eggs, because the cells have so little yolk, cleavage is
holoblastic (from the Greek “holo,” meaning “whole” and “blastos,” meaning “germ”). In
holoblastic cleavage, each cleavage extends all the way through the egg, completely dividing it.
A sea urchin (Phylum Echinodermata) has isolecithal eggs and
holoblastic cleavage. Each cleavage completely divides the egg.
Holoblastic Cleavage
Linear Mode
