Fish, Amphibian, Bird and Reptile Placentation

The fishes are non-amniotes, which have evolved a wide variety of modifications for successful viviparity (Amoroso 1960; Hogarth 1976; Wourms 1981; Mossman 1987). The most widely used system is an increased secretion of nutrients into the ovarian or uterine cavity and absorption via the fetal gut or skin surface, but a number of genera have produced specialized structures, both embryonic and extraembryonic, some of which closely parallel amniote placentas.