Monotreme and Marsupial Placentation

Prototheria (e.g. Platypus) lay eggs and Metatheria (e.g. Kangaroo) produce shelled eggs in utero with the same reliance as birds on a bi- or trilaminar yolk sac and allantois for mobilization of yolk and respiratory exchange. However, they all have far less yolk and a much earlier hatching or birth than the birds and require subsequent mammary nutrition and in Metatheria, protection in a pouch (Luckett 1976, 1977; Tyndale-Biscoe and Renfree 1987). A few marsupials lose their shell during development in utero and show localized invasion of the uterine epithelium by the yolk sac trophectoderm, and other families develop an area of chorioallantoic placentation in the last quarter of the typically very short 12–14 day gestation.