In animals, one can usually distinguish 4 stages of embryonic development. Cleavage Patterning Differentiation Growth Cleavage Mitosis and cytokinesis of the zygote, an unusually large cell, produces an increasing number of smaller cells, each with an exact copy of the genome present in the zygote. However, the genes of the zygote are not expressed at first.
Cleavage ends with the formation of a blastula .
Patterning During this phase, the cells produced by cleavage organize themselves in layers and masses, a process called gastrulation . There is little visible differentiation of the cells in the various layers, but probes for cell-specific proteins reveal that different groups of cells have already started on specific paths of future development. By gastrulation, the genes of the zygote genome are being expressed. View these stages as they occur in amphibians such as Xenopus laevis, the South African clawed frog. In time, the cells of the embryo differentiate to form the specialized structures and functions that they will have in the adult.