Which extra-embryonic membrane has a well-developed vascular structure that contributes to the vascular system of the developing mammalian embryo?
A - The chorion
B - None of them
C - The allantois
D - The amnion
E - The yolk sac
Answer: E - The yolk sac
The YOLK SAC in the mammals doesn't contain any yolk, but maintained a well-developed vascular system and contributes to the vascular system of the embryo.