BLACKBURN, D.G.; JOHNSON, A.R.; PETZOLD, J.L. ; Trinity College, Hartford: Fetal membrane ultrastructure in the corn snake, Pantherina guttatus

In amniotes, fetal (extraembryonic) membranes are responsible for all physiological exchange between eggs and their environments. The anatomical basis for such exchange is largely unknown in reptiles, and has not previously been studied in oviparous snakes. We used transmission EM and thin-section histology to examine fetal membranes in oviparous corn snakes. During development, the chorioallantois shows an increase in vascularity and a decrease in thickness of the chorionic epithelium. These features enhance respiratory exchange across the eggshell. As in viviparous snakes, the allantois invades the yolk cleft, vascularizing the abembryonic pole of the egg and forming an omphalallantoic membrane. As development proceeds, the isolated yolk mass is digested by the omphalopleure, converting the omphalallantoic membrane into a secondary chorioallantois; thus the entire egg becomes surrounded by vascularized membrane. Both the chorioallantois and the omphalopleure show evidence of phagocytosis of eggshell material. Coupled with recent studies of eggshell morphology and calcium mobilization, these findings offer evidence for the uptake of eggshell material by the fetal membranes. In viviparous squamates, placental transfer of organic and inorganic nutrients is widespread, if not universal. The capacity for uptake by the fetal membranes may well be a primitive sauropsid feature that long predates the evolution of viviparity.