Pregnancy

Development of Embryo and Fetus

The Embryo
The word embryo (Greek: swelling within) refers to the growing organism from the second to the eighth week of its life. During this time, it develops from a tiny cell cluster into a little growth of about 1 inch in length. As this development proceeds, the placenta, a special organ of interchange, begins to grow between the embryo and the uterus. The embryo is connected to the placenta by the umbilical cord. (Soon after the birth of the baby, its umbilical cord is still connected to the placenta which is then expelled from the uterus. For this reason, the placenta is also called the afterbirth.) The placenta acts as a filter and as a barrier. It allows the embryo (and later the fetus) to absorb food and oxygen from the woman's blood and to eliminate carbon dioxide and other waste from its own blood in return. At the same time, however, the two blood systems remain completely separate.
During the first month of its life, the human embryo looks like that of any other higher animal, such as a cat, dog, or pig, for example. Then, during the second month, it slowly assumes human features. It starts to develop a recognizable face, as well as arms, legs, fingers, and toes. Between its legs, the primitive beginnings of sexual organs become discernible, although they are still undifferentiated at this point (i.e., they are the same for both male and female). When the entire growing organism finally becomes clearly identifiable as human, it leaves the stage of the embryo and enters that of the fetus.