From the abdomen of the embryo arises an organ, the _allantois_, which
is destined to carry the blood-vessels of the embryo to the placenta,
and at the same time to give rise to the formation of the latter. In
the placenta the blood-vessels of the embryo are separated from those
of the mother by walls so thin that the nutritive juices of the
maternal blood transude into the venous blood of the embryo, as well
as combined oxygen in the blood necessary for its respiration. Up to
this point the vitellus of the egg, nourished by endosmosis through
its membranes, had sufficed for the nutrition of the still very small
embryo. While these phenomena are taking place, and while the
substance of the two conjugated germs divides into an ever increasing
number of cells, which become differentiated in layers to form the
future organs (Fig. 21), while certain groups of cells are prepared
some to form the intestinal canal, others the muscles and blood
vessels, others the skin and organs of sense, others arising from the
last to form the brain, the spinal cord and the nerves, the mother can
still live her ordinary life. She suffers, however, from different
disorders connected with what is passing on in her body.

It is a curious fact that these disorders are more accentuated at the
commencement of pregnancy, when the womb is hardly enlarged, than at
the end. They consist chiefly of nervous troubles--slight derangement
of the cerebral functions and sensations, etc. Obstinate vomiting,
peculiar desires, and changes of temper are some of the most frequent
troubles of pregnant women, and probably arise more from local nervous
irritation than from general transformations of the nutrition of the
body. The mother's body is becoming adapted to the development of the
infant in the womb. However embarrassed a woman may be in the last
months of pregnancy by the great swelling of the belly (Fig. 22) the
disorders are less accentuated than at the beginning of pregnancy.

During pregnancy menstruation ceases. The sexual appetite is very
variable; in many pregnant women it is diminished, in others there is
no change, and it is seldom increased. There are other troubles which
are more or less frequent, such as varicose veins in the legs caused
by pressure of the uterus on the veins.

But all the sufferings of pregnancy and childbirth are compensated for
by the ardent desire of the normal woman to have a child, and by the
happiness of hearing its