There is more to a developing reptile or mammal than the embryo itself. The embryos of these vertebrates are surrounded by several extraembryonic membranes that originate from the embryo but are not part of it. Extraembryonic membranes function in nutrition, gas exchange, and waste removal. In mammals they interact with tissues of the mother to form the placenta. The evolutionary relationships between the extraembryonic membranes of birds and mammals were discussed in Key Concept 32.4.
focus your learning
The four extraembryonic membranes in birds are the yolk sac, the allantoic membrane, the amnion, and the chorion.
The placenta forms from the embryonic chorion and tissues of maternal uterine wall.
During the first trimester, the embryo is most sensitive to environmental effects.