First a sperm has to fuze with an egg, in order to become fertilized. Sometimes the sperm has a difficult time getting through the egg's shell so the sperm goes through acrosome reaction. In this process a reaction occurs in acrosome (cap-like structure) of the sperm as it begins to approach the egg. The surrounding membrane of the acrosome fuses with the plasma membrane of the sperm which exposes what is inside the acrosome; such as enzymes that break down the shell and allow fertilization.

A cortical reaction is when a sperm cell unites with an egg's plasma membrane. This ensures that no other sperm can fertilize this particular egg.

Cleavage begins in a zygote cell. The zygote cell goes under a series of rapid mitotic divisions. There is no growth however there is DNA synthesis. Cleavage is complete with the formation of a blastula (a hallow ball of cells with a liquid cavity).

Gastrulation is when the blastula gets transformed into a gastrula containing 3 layers of cells. The ectoderm is the outermost layer, forming skin, hair, nerves, and sweat glands. The mesoderm is the middle layer which helps form things associated with movement and support, such as muscles, organs, bone and cartilage. The endoderm is the innermost layer which helps create inner lining, tissues, organs of the digestive system. The archenteron is a new cavity located inside the endoderm.

Organogenesis occurs right after gastrulation, it is a process by which the 3 layers develop into organs.