This page discusses embryo development in reptiles, birds, and mammals, focusing on four extraembryonic membranes: amnion, yolk sac, chorion, and allantois, which support the embryo. It categorizes ma...This page discusses embryo development in reptiles, birds, and mammals, focusing on four extraembryonic membranes: amnion, yolk sac, chorion, and allantois, which support the embryo. It categorizes mammals into monotremes, marsupials, and placental mammals, highlighting that humans belong to the placental group, where the placenta connects the embryo to the mother.
The yolk sac stores enough food for the short period of time the embryo remains in the uterus. Even after the offspring is big enough to leave the pouch, it may often return to the pouch for warmth an...The yolk sac stores enough food for the short period of time the embryo remains in the uterus. Even after the offspring is big enough to leave the pouch, it may often return to the pouch for warmth and nourishment. In marsupials, the short period of development within the mother’s uterus reduces the risk of her immune system attacking the embryo. Another pro is that the mother can expel the embryo from her pouch if she is pursued by a predator or if food is scarce.
