Once the zygote attaches to the wall of the uterus, it is known as the embryo. During the embryonic phase, which will last for the next 6 weeks, the major internal and external organs are formed, each beginning at the microscopic level, with only a few cells. The changes in the embryo’s appearance will continue rapidly from this point until birth.
While the inner layer of embryonic cells is busy forming the embryo itself, the outer layer is forming the surrounding protective environment that will help the embryo survive the pregnancy. This environment consists of three major structures: The amniotic sac is the fluid-filled reservoir in which the embryo(soon to be known as a fetus)will live until birth, and which acts as both a cushion against outside pressureand as a temperature regulator. The placenta is an organ that allows the exchange of nutrients between the embryo and them other, whileat the same time filtering out harmful material. The filtering occurs through a thin membrane that separates the mother’s blood from the blood of the fe tus, allowing them to share only the material that is able to pass through the filter. Finally, the umbilical cord links the embryo directly to the placenta and transfers all material to the fetus. Thus the placenta and the umbilical cord protect the fetus from many foreign agents in the mother’s system that might otherwise pose a threat.