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The left ovary of the bonnethead shark, Sphyrna tiburo, is rudimentary, and the right ovary supplies both oviducts which share a common ostium situated in the falciform ligament. Preceding ovulation the nidamental gland of each oviduct hypertrophies and the caudal two-thirds of each oviduct is modified to form a uterus. In the Florida-Caribbean area Sphyrna tiburo probably mates in March and 3-7 eggs are fertilized in the vicinity of the nidamental gland of each oviduct.The developing embryo is nourished during the first 3-4 months of gestation by yolk stored in its extensive yolk sac. Approximately three and one-half months after fertilization, the distal portion of the yolk sac becomes convoluted and interdigitates with deep folds in the uterine wall to form a yolk-sac placenta. As the placenta develops, the maternal uterine epithelium is reduced from columnar cells to squamous cells, and the foetal yolk-sac epithelium is reduced from columnar and cuboidal cells to squamous cells. Exchange between the maternal and foetal blood systems takes place through maternal endothelium, reduced maternal epithelium, egg-case membrane, reduced foetal epithelium, and foetal endothelium.

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JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. American Society of Ichthyologists and Herpetologists (ASIH)is collaborating with JSTOR to digitize, preserve and extend access to Copeia.Elasmobranchs demonstrate diverse means of nurturing their embryos, there being within this vertebrate group oviparous, ovoviviparous, and truly viviparous, or placental, species. Carcharhinus falciformis is of the latter type. Observations, gross and microscopic, made on a single uterus of this species and its contained embryos and placentae are presented.All elasmobranch placentae are of the yolk-sac type, but within this single classification rather wide structural variations occur. The placenta of C. falciformis possesses a discoidal shape and is completely nondeciduate. The following layers intervene between the foetal and maternal blood systems: (1) maternal endothelium, (2) maternal epithelium (essentially not reduced), (3) possibly the egg-case membrane, depending on factors stated in the paper, (4) foetal epithelium (greatly reduced), and (5) foetal endothelium.