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 _Koelliker_.)
_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 vertebrae 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 _Koelliker_).
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.

In the placenta the embryonic and maternal vessels without actually
communicating, are placed in intimate contact, which allows nutritive
matter and oxygen to pass by endosmosis from the maternal vessels to
those of the embryo. Figure 21 shows a human embryo at the beginning
of the fifth week of pregnancy.

[Illustration: FIG. 22. Sagittal section of a primipara in the
last month of pregnancy.]

=Duration of pregnancy. Birth.= Pregnancy lasts from conjugation,
which is synonymous with conception, til