Embryonic development in viviparous teleosts occurs within the ovary as intraovarian gestation that is unique among vertebrates, because all teleosts lack Müllerian ducts from which the oviducts develop in the other vertebrates. Lacking oviducts, viviparous teleosts have evolved intraovarian gestation. Both insemination and fertilization occur in the ovary. The analysis of ovaries of Xenotoca eiseni (Goodeidae) in non-gestation and gestation stages reveals the features of viviparity coincident with intraovarian gestation: fertilization is intrafollicular; during cleavage, the embryos are discharged from the follicle into the lumen. Gestation is intraluminal; sequentially the embryos develop two types of nutrition: lecithotrophy (initially nourishment by yolk) which changes to matrotrophy (nourishment from maternal source) when the embryo develop extensions of the perianal region, the trophotaenia, which absorb the nutrients from the ovarian lumen.