cell created by the union of a female gamete (ovum) with a male gamete (spermatazoan), with a full set of paired chromosomes, and undergoing cell division (growing); in birds, the female is heterogametic, with dissimilar ZW sex chromosomes, the male is homogametic, with similar ZZ sex chromosomes, so the female gamete provides either the male or the female chromosome and thus determines the sex of the zygote—the opposite of the case in mammals; becomes an embryo when cell maturation and differentiation begin shortly after fertilization