Fertilization and Cleavage

Abstract

The process of fertilization can be defined either on an organismal or on a cellular level. At the organismal level, fertilization is equivalent to mating and involves the complex behaviors associated with a species’ mating system. At the cellular level, it is the phenomenon by which an oocyte or ovum is mobilized to begin embryonic development. Fertilization is commonly thought to be the process by which male and female sex cells fuse. This commonly held notion, however, is misleading. It is more accurate to think of vertebrate fertilization in general as the activation of an oocyte or ovum by some means. With the exception of some unisexual vertebrates,¹ this activation involves the fusion of maternal and paternal genomes.²