The the frugal cellular slime mold, consisting of only a few cell types, seldom indulges in sex, instead undergoing repeated cycles of asexual reproduction. Chapter 4 shows you how to design experiments that use this organism to explore principles of development operating in the realm of simple organization. In organisms that do use sexual reproduction, gametogenesis marks the beginning of development. Chapter 6 invites you to explore questions surrounding the mechanisms of this event using sea urchin gametes. The journey started at fertilization continues at breakneck speed through cleavage.
Cleavage provides the embryo with a large number of cells for continuing the trip.
Some of these messages instruct the cells to don new clothes, in the form of new cell-surface molecules. Once they put on their new clothes, the cells can't sit still. They begin moving to new positions in the process of gastrulation, which brings the germ layers into positions appropriate for their specific fates. You will see how these movements are choreographed in the sea urchin, fruit fly, and chick in Chapters 6 through 10. With the germ layers in position, the trip enters a phase of dynamic interactions known as organogenesis.