<P> 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 development of a multicellular embryo . </P> <P> 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 . </P> <P> 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 taxa show different cellular and acellular envelopes englobing the vitelline membrane . </P> <P> 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 fertilisation, or outside in the case of external fertilisation . The fertilized egg cell is known as the zygote . </P>

The development of an unfertilized egg is termed