The formation of fetal membranes in mammals is a remarkable process, crucial for nourishing and protecting the developing embryo. These membranes – the amnion, chorion, allantois, and yolk sac – arise from the embryo's extraembryonic tissues.

Initially, after implantation, the trophoblast cells, which form the outer layer of the blastocyst, proliferate. From the inner cell mass, two primary germ layers, the epiblast and hypoblast, differentiate. The amnion is the first to form, originating from the epiblast. A cavity, the amniotic cavity, develops within the epiblast, and folds of the somatopleure (a layer formed by ectoderm and somatic mesoderm) rise over the embryo, eventually fusing to enclose it in a sac. This sac fills with amniotic fluid, providing a shock-absorbing environment and preventing adhesion.

Simultaneously, the yolk sac develops from the hypoblast and splanchnic mesoderm. In mammals, it's largely vestigial in terms of nutrient supply, but plays roles in early hematopoiesis and germ cell migration.

The allantois grows out from the hindgut as a diverticulum, composed of endoderm and splanchnic mesoderm. It expands into the extraembryonic coelom. Its primary function is to collect embryonic waste and, more significantly, its blood vessels contribute to the umbilical circulation, connecting the embryo to the developing placenta.

Finally, the chorion is the outermost membrane, formed from the trophoblast and somatic mesoderm. As the amnion expands, the chorion also grows, eventually surrounding the entire embryo and other membranes. The allantois often fuses with the chorion to form the chorioallantoic membrane, which is the primary site of nutrient and gas exchange, forming the fetal component of the placenta. The chorion’s villi interdigitate with the maternal uterine wall, establishing the critical interface for pregnancy.