In placental mammals, the first extraembryonic membrane to form is the trophoblast (see Figures 43.4C and 43.5). When the blastocyst reaches the uterus and hatches from its encapsulating zona pellucida, trophoblast cells interact directly with the endometrium. Adhesion molecules expressed on the surfaces of these cells attach them to the uterine wall. By secreting proteolytic enzymes, the trophoblast burrows into the endometrium, beginning the process of implantation. Eventually the entire trophoblast is within the wall of the uterus. The trophoblast cells send out numerous projections, or villi, to increase the surface area of contact with maternal blood.
Meanwhile, the hypoblast cells proliferate to form what in the bird would be the yolk sac. But there is virtually no yolk in eggs of placental mammals; instead the yolk sac contributes mesodermal tissues that interact with trophoblast tissues to form the chorion. The chorion, along with tissues of the uterine wall, produces the placenta, the organ that exchanges nutrients, respiratory gases, and metabolic wastes between the mother and the embryo (Figure 43.16).
At the same time the yolk sac is forming from the hypoblast, the epiblast produces the amnion, which grows to enclose the entire embryo in a fluid-
An allantois also develops in mammals, but its importance depends on how well nitrogenous wastes can be removed by the blood vessels of the placenta. The human allantoic sac is small because the human placenta deals effectively with the fetal nitrogenous wastes. In contrast, the pig placenta is less effective at clearing nitrogenous wastes from the fetus, so the pig’s allantoic sac is large. In humans and other placental mammals, allantoic tissues contribute to the formation of the umbilical cord, by which the embryo is attached to the chorionic placenta (see Figure 32.19B). It is through the blood vessels of the umbilical cord that nutrients and oxygen from the mother reach the developing fetus, and wastes, including carbon dioxide and urea, are removed.