Examples of three types of eggs arc shown in the figure on the handout. At fertilization, the enzymes in the acrosome of the sperm help to penetrate the egg. This process requires that the sperm break through the plasma membrane as well as the vitelline membrane around the egg. Holoblastic cleavage occurs when the cleavage furrows pass through the entire egg. Such cleavage can either be equal, where the resulting cells contain the same amount of yolk, or unequal, in which some cells contain more yolk than others.
Meroblastic cleavage occurs more in macrolecithal eggs.
In this form, cleavage takes place only in a disk at the animal pole, and the cleavage furrows do not extend into the yolk. Such cleavage results in the formation of the blastodisk that lies on top of the yolk. In placental mammals, holoblastic equal cleavage occurs, forming a ball of cells called the morula. The trophoblast will be the fetal contribution to the placenta, while the inner cell mass develops into the embryo. Gastrulation and formation of germ layers Blastulation is characterized by cell division and rapid increase in the number of cells in the developing embryo.