Raven Johnson Biology Chapter 17 Answers to Review Questions 1.
Cleavage is the initial division of a fertilized egg.
It proceeds so rapidly that the zygote does not increase in size. A blastula is a hollow ball of cells formed from the rapid cleavage divisions of the zygote. As cell division progresses, the blastula invaginates to form a gastrula. A gastrula is an invaginated blastula that continues in rapid cell division. The wall of cells that invaginates and becomes an internal fold is the endoderm, destined to become the gut.