In some cases, the vegetal pole is thought to differentiate into the extraembryonic membranes that protect and nourish the developing embryo, such as the placenta in mammals and the chorion in birds.
This passage describes how the vegetal pole of an embryo, which develops into the extraembryonic membranes that provide protection and nourishment for the developing embryo. The specific examples are the placenta in mammals and the chorioallantoic membrane in birds.