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.
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 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.
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.