At the age of four weeks, the embryo is about the size of a pea. A primitive heart is beating, the head is defined with rudimentary eyes and ears, and tiny bumps represent arms and legs. The embryo also contains a primitive nervous system, and the head has begun to enlarge. A cartilage skeleton has appeared, and muscles have taken shape.
By the end of eight weeks, the embryo is somewhat human looking. Facial features are evident, and most of the organs are well developed. From this point onward, development consists chiefly of growth and maturation. The embryo is about 1.5 inches in length. Henceforth it is known as a fetus.
Nourishment of the embryo, and then the fetus, is accomplished through the placenta. The maternal and embryonic blood supplies meet at this organ, but the blood does not mix. Instead, diffusion accounts for the passage of gases, nutrients, and waste products across the membranous barriers. The placenta is also an endocrine gland because it secretes estrogen and progesterone to continue to inhibit follicle development and maintain the integrity of the endometrium. As the embryo becomes a fetus, it moves away from the placenta, and a length of tissue called the umbilical cord becomes its source of attachment to the maternal blood supply.