Three principal modes of embryonic nutrition occur in the vertebrates, viz., 1) uitellogenesis, the sequestering of yolk, synthesized in the liver, in the oocyte; 2) histotroph, nutrient substances supplies to the embryo in viviparous animals by secretory activity of the uterus; or 3) placenta, the direct provision of nutrients by a maternal-fetal connection. Viviparous placental sharks sequentially utilize all three methods of embryonic nutrition to provide sustenance to their developing young. The progressive shift from reliance on yolk, to histotroph, to the placenta allows for the study of multistep phenomena in a single animal. Placental sharks are very suitable models for the study of comparative vertebrate reproductive biology owing to the similarity to mammals in basic endocrine regulatory factors. The shark yolk sac probably represents the most primitive typical vertebrate fetal membrane. Although homology is not direct with the placentae of mammals, the same basic endocrine hormones govern placental development in both groups. Further, the yolk sac and placenta of sharks may be regarded as the "common denominator" vertebrate fetal membrane. We have identified a model elasmobranch species, the Atlantic sharpnose shark, R hizoprionodon terraenouae, in which the three major modes of embryonidfetal nutrition can be studied. The gravid adult female is 110 cm in length and usually contains 4-6 young per litter. Size of young at birth is 29-37 cm. (Compagno, '84). Parsons ('83) has presented a field study of its reproductive biology. This model would provide in a single species the opportunity to correlate and experimentally verify the role of steroid hormones in the regulation of yolk protein production during gestation, the transuterine transfer of proteins, and the development of hormonesensitive uterine sites for placental attachment. Preliminary studies (Hamlett, unpublished), using radioimmunoassay procedures, have found measurable levels of estradiol and testosterone in the ovary, uterus, and placenta of sharks. Ultra-