ABSTRACT
Evolution of placentation involves modifications of existing maternal and fetal membranes. The ontogenetic transformation of the shark oviduct into a functional uterus for the retention and metabolic support of developing young is regulated by endocrines (Chapter 9 of this volume). ln the developing fetus, the yolk sac and stalk display terminal differentiation into a regionally specialized placenta and an umbilical cord respectively. Accessory vascular structures of the umbilical cord, termed appendiculae, are present in some species where they may serve a nutrient absorptive function as well as contribute secretions to periembryonic fluids (Southwell and Prasad 1919; Mahadevan 1949; Hamlett 1993; Hamlett et a/. 1993a, b, c).
