Stewart, J.R., D.G. Blackburn, D.C. Baxter, and L.H. Hoffman (1990). Nutritional provision to the embryos in Thamnophis ordinoides (Squamata: Colubridae), a predominantly lecithotrophic placental reptile. Physiological Zoology 63: 722-734.

Abstract: Quantitative analysis of the composition of eggs and their sibling neonates in the viviparous natricine snake Thamnophis ordinoides revealed that yolk provided the principal source of organic nutrition but that embryos received a substantial allotment of inorganic nutrients from the placentas. The placental provision of water and sodium equaled or exceeded yolk supplies, and placental transport accounted for 23% of neonatal calcium composition. There was no difference between egg and newborn quantities of total phosphorus or total potassium., whereas neonates contained less total magnesium than eggs. The mode of embryonic nutrition in this species is characterized as predominantly lecithotrophic, yet placental nutrient provision contributes significantly to embryonic nourishment. Placental transport of sodium and embryonic uptake of water was greater in recently ovulated eggs that contained relatively low levels of sodium and water respectively. thus, placental sources compensated for low yolk provision. Placental transport of calcium was independent of yolk calcium content and correlated positively with neonatal calcium content. This pattern of provision, in which placental sources determine neonatal content independent of egg content, has been described as facultative placentotrophy. A similar embryonic nutritional pattern was recognized previously in another predominantly lecithotrophic natricine snake.

Thamnophis sauritius