Characteristic feature is unicellularity.
Fertilization is the procession of events that begins when a spermatozoon makes contact with an oocyte or its investments and ends with the intermingling of maternal and paternal chromosomes at metaphase of the first mitotic division of the zygote. Fertilizationrequires slightly longer than 24 hours. Takes place in the ampulla of the uterine tube. The formation of male and female pronuclei. The beginning of the first mitotic division, or cleavage, of the zygote.