Blastulation

Blastulation
Cleavage, however modified by different cleavage patterns and by the presence of varying amounts of yolk, results in a cluster of cells called a blastula (commonly called a blastocyst in mammals) (Figure 8-11). In many animals the cells arrange themselves around a central fluid-filled cavity called the blastocoel. At this point, the embryo consists of a few hundred to several thousand cells poised for further development. There has been a great increase in total DNA content, since each of the many daughter cell nuclei, by chromosomal replication at mitosis, contains as much DNA as the original zygote nucleus. The whole embryo, however, has not increased in size above the zygote; it has been subdivided into smaller and smaller cells.