=The womb. The placenta.= The womb or uterus is the size of a small egg flattened in one direction. It terminates below in the neck or _cervix_, which is prolonged into the vagina as a projection, called the vaginal portion of the uterus. The cavity of the womb is continued into the neck and opens below in the vagina by an aperture which is round in virgins and is called the external _os uteri_. The walls of the womb consist of a thick layer of unstriped muscle. When childbirth takes place it causes tearing which makes the external os uteri irregular and fissured. During copulation the aperture of the penis or male organ is placed nearly opposite the os uteri, which facilitates the entrance of spermatozoa into the uterus. (For the illustration of these points see Fig. 18.) The vitellus and the membrane of the egg enlarge with the embryo and absorb by endosmosis the nutritive matter necessary for the latter, contained in the maternal blood. The womb itself enlarges at the same time as the embryo. [Illustration: FIG. 20. Human egg of the second week: magnified eight times. (After _Kölliker_.) _Chor._ Chorion or envelope of the egg. _Vill._ Villi of the chorion. _Emb._ Embryo (near the head are seen the branchial arches). _Umb._ Umbilical vesicle. _Am._ Amnion.] The fasciculus attached to the embryo is the allantois which becomes the umbilical cord. The vertebræ are already easy to recognize in this embryo. The embryo is formed from a portion of blastoderm, that is to say, from the cellular layer applied to the membranes of the egg and arising from the successive divisions of the two primary conjugated cells and their daughter cells. The embryo has the form of a spatula with the head at one end and the tail at the other. From its walls is detached a surrounding vesicle (Fig. 20) called the _amnion_, while another vesicle, the _umbilical vesicle_, grows from its ventral surface and serves, in birds, for the vitelline circulation of the egg which is detached from the mother's body. In man, the umbilical vesicle is unimportant. In its place the circulation of the blood takes place by the aid of another vesicle, called the _allantois_, which arises from the intestine of the embryo, and which becomes attached to the walls of the womb in the form of a thick disk called the placenta. [Illustration: FIG. 21. Embryo of four weeks (After _Kölliker_). 1. Auditory vesicle. 2. Ocular vesicle. 3. Olfactory fossa. 4. Bud forming upper maxilla. 5. Bud of lower maxilla. 6. Right ear. 7. Liver. 8. Upper limb. 9. Lower limb. 10. Caudal extremity.] The placenta is formed of dilated blood vessels which meet the maternal blood vessels, also dilated, in the uterine wall, allantois later on becomes the umbilical cord.