Placentation in the garter snake, Thamnophis sirtalis

The placental memebranes and uterus of the garter snake, Thamnophis sirtalis , were studied using histological, histochemical, electron microscopic, dye transfer, and radioisotopic techniques. The conceptuses are completely enclosed throughout gestation by a transparent shell membrane which is produced by glandular epithelia in the uterine segment of the oviduct. Both chorio‐allantoic and omphalo (yolk‐sac) placentation are observed in this snake. The growth of the extra‐embryonic mesoderm takes place in a manner peculiar to placental reptiles, and results in the isolation of the omphaloplacenta from the yolk‐sac wall. On the basis of morphology, enzyme histochemistry, and phagocytosis of Trypan blue particles, this structure is interpreted as a site of histiotrophic absorption. The chorio‐allantoic region of placentation is simple in structure. Fetal and maternal capillaries are closely apposed but always separated by layers of uterine and chorionic epithelium and the thin shell membrane. The placental membranes of the garter snake are similar in many respects to those of other live‐bearing snakes, but less specialized than most lizard placentate. Isotopically labelled sodium and glycine are passed to the fetus following maternal injection, the latter at least apparently via the omphaloplacenta. The permeability to iron and phospate is extremely low. On the basis of these results and the selective transfer of certain dyestuffs, it appears that the shell membrane functions as a dialyzing membrane.