Embryogenesis

Embryogenesis is the process by which the embryo is formed and develops, until it develops into a fetus.

Embryogenesis starts with the fertilization of the ovum (or egg) by sperm. The fertilized ovum is referred to as a zygote. The zygote undergoes rapid mitotic division with no significant growth (a process known as cleavage) and cellular differentiation, leading to development of an embryo.

Although embryogenesis occurs in both animal and plant development, this article addresses the common features among different animals, with some emphasis on the embryonic development of vertebrates and mammals.

Fertilization and the zygote

The egg cell is always asymmetric, having an "animal pole" (future ectoderm and mesoderm) and a "vegetal pole" (future endoderm). It is also covered with different protective envelopes, with different layers. The first envelope - the one in contact with the membrane of the egg - is made of glycoproteins and is known as the vitelline membrane (zona pellucida in mammals). Different taxa show different cellular and acellular envelopes englobing the vitelline membrane.

Fertilisation<!-- The spelling fertilization is a British English variant. The spelling fertilization is also used, and is the official spelling in American and Canadian English. See . --> (also known as 'conception', 'fecundation' and 'syngamy'), is the fusion of gametes to produce a new organism. In animals, the process involves a sperm fusing with an ovum,...
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