Embryogenesis is the process by which the embryo forms and develops. In mammals, the term refers chiefly to early stages of prenatal development, whereas the term fetus and fetal development describe later stages.
Embryogenesis starts with the fertilization of the egg cell (ovum) by a sperm cell, (spermatozoon). Once fertilized, the ovum is referred to as a zygote, a single diploid cell. The zygote undergoes mitotic divisions with no significant growth (a process known as cleavage) and cellular differentiation, leading to the development of a multicellular 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.
The egg cell is generally asymmetric, having an “animal pole” (future ectoderm and mesoderm) and a “vegetal pole” (future endoderm). It is covered with 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 taxes show different cellular and cellular envelopes englobing the vitelline membrane.
Fertilization (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, which eventually leads to the development of an embryo. Depending on the animal species, the process can occur within the body of the female in internal fertilization, or outside in the case of external fertilization. The fertilized egg cell is known as the zygote.