Abstract
All the early development in eumetazoans, from egg fertilization through cleavage, directed cell migration, formation of germ cell layers to the formation of the operational central nervous system at the phylotypic stage, is epigenetically regulated by parental cytoplasmic factors, not by zygotic genes. The early development depends on epigenetic factors alone for different periods of time in different species. This period of exclusive epigenetic control and regulation of the early development varies from the two-cell stage in some mammal species to ~5000-cell stage in Xenopus. Despite the fact that after these stages zygotic genes start to be expressed, epigenetic parental factors still continue to influence the early development until the phylotypic stage by regulating expression of zygotic genes.
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