| After implantation into the uterine endometrium, the developing embryo becomes surrounded by the chorionic, amniotic, and allantoic membranes. In fish and birds these same membranes allow diffusive exchange of of CO2 and O2 with the environment, while nutrients are provided by the yolk sac. In mammals, nutrients and wastes are exchanged with the maternal circulation across a specialized region of these these membranes called the placenta. The placenta provides a large surface area of close association between the two independant circulations for diffusive and carrier mediated exchange. In addition the placenta produces hormones to support the pregnancy. The gross arrangement of the placenta may be diffuse, zonary discoid, or cotyledonary, depending on the species. |
