Abstract
The discovery of primitive placentae and well-defined corpora lutea in the viviparous snake Storeria by the writer has prompted a preliminary survey of these structures in other live-bearing snakes. Further, an attempt has been made to ascertain the function of the corpus luteum. Of special interest is the possibility of homologizing the structure and function of these organs with those of the mammal and of determining their rôle in reptilian gestation.
Although placentation in the lizards and snakes 1 , 2 , 3 and corpora lutea in lizards 4 , 5 have been described for certain European and Australian forms, they are now recorded for the first time in our North American reptiles. In certain species of the genera Storeria, Potamophis, and Thamnophis, a yolk-sac placenta exists during the earlier stages of pregnancy; it is more developed in the first 2 of these genera. “Implantation” is brought about by a constriction of the extremely thin, transparent uterus between each embryo giving the gravid uterine horn the appearance of a linked sausage. On the ventral side of the uterus which encases the large yolk-sac, is an elliptical, creamy-white area which marks the extent of the yolk-sac placenta. In this region both uterine epithelium and the opposing ectodermal chorion of the foetus have given rise to very large columnar cells which cytologically suggest an active exchange of substances. The modified maternal and foetal tissues of this circumscribed area lie in close contact with each other and correspond to similar yolk-sac placentae described by Weekes 3 for Australian reptiles. Beyond this area the chorio-allantoic and the uterine wall are highly vascularized. In the genus Storeria the uterine epithelium is partly eroded, allowing the maternal capillaries to make direct contact with the chorio-allantoic membrane.
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