BLACKBURN, D.G.*; JOHNSON, A.R.; PETZOLD, J.L.: Histology of the extraembryonic membranes of the oviparous corn snake, Elaphe guttata.

The extraembryonic membranes of amniote eggs serve vital nutritional and respiratory functions throughout development. However, very little is known about the structure and function of these membranes in oviparous reptiles. We examined extraembryonic membranes of the corn snake Elaphe guttata (Colubridae), using resin – embedded tissues sectioned on glass knives. Following oviposition, the embryos develop a chorioallantois as well an omphalallantoic membrane. The chorioallantois is highly vascularized, and its capillaries are separated from the inner lining of the eggshell by a bilayered epithelium that progressively thins during development. Small basophilic granules, possibly taken up from the eggshell, accumulate in apices of the epithelial cells. Depletion of yolk leads to expansion of the chorioallantois at the expense of the yolk sac. The omphalallantoic membrane forms through penetration of the allantois into the yolk cleft. The isolated yolk mass (IYM) undergoes progressive reduction, transforming into isolated patches of yolk droplets that are underlain and surrounded by the allantois. Yolk of the IYM appears to be digested by the bilaminar omphalopleure as well as by the allantois and motile cells with phagocytic capabilities. Diminution of the IYM and the overlying epithelium lead to close approximation of the allantoic capillaries to the eggshell throughout the abembryonic hemisphere of the egg. Consequently, for the last weeks of development, the egg is surrounded by thin vascularized membranes that presumably function in respiratory exchange. To better understand their functional morphology, we currently are examining cell ultrastructure of these membranes with electron microscopy.