Embryogenesis is the process by which the embryo is formed and develops, until it develops into a fetus. It 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 divisions 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 (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 fertilisation, or outside in the case of external fertilisation.
The fertilized egg cell is known as the zygote.
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