In vertebrates, there are three principal modes of nutrition for developing embryos: (1) the laying down of food stores, synthesized in the liver, in the oocyte (vitellogenesis) (this is the most common mode in nonmammalian species); (2) the direct provision of nutrients to the developing young through the intermediary of the uterine wall by either secretory activity (histotroph) or (3) a direct maternal connection (placentation) (Table I). In egg-laying vertebrates, yolk is the only source of fetal nutrition; however, in live-bearing (viviparous) species, at least two of these mechanisms may exist in an individual. Thus, aplacental rays and torpedos have eggs with a very small amount of endogenous yolk, sufficient only for early development. The bulk of the nutrients are supplied as ‘‘histotroph”* by specialized secretory elements of the reproductive tract. In ‘‘placental” elasmobranchs (some sharks), yolk is present and may be reduced in amount, but the yolk-sac placenta forms a site for exchange of nutrients from the mother to the fetus. This is also true of placental reptiles (certain small lizards and snakes). However, in reptiles both a yolk-sac placenta (choriovitelline or omphaloplacenta) and the phylogenetically newer chorioallantoic placenta exist and function sequentially depending on the quantity of endogenous yolk supplied with the egg. In eutherian mammals, no true yolk is present in the eggs, the developing young being dependent on histotroph from the secretory epithelium for variable times depending on the species (Table II).