Once the sperm and egg nuclei fuse, fertilization is accomplished. The zygote travels down the oviduct and eventually implants in the endometrium. It then secretes human chorionic gonadotropin, the presence of which is the basis for pregnancy tests. The size of the cell does not increase during this stage. Morphogenesis is the reshaping of the embryo as cells migrate to different areas.
Differentiation occurs as cells take on specific structures and then functions.
Growth accompanies cell division during embryonic development. During cleavage, the mass of cells is referred to as a morula. The blastula stage begins once the morula transforms into a hollow ball with an inner cell mass off to one side. The bilayered disk elongates, and a primitive streak is seen at the midline of the embryo. Some of the cells within the primitive streak invaginate, giving rise to a third germ layer, the mesoderm.