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EVOLUTION OF THE F02TUS. 175
CHAPTER XII.
EVOLUTION OF THE FOITUS.
The ovum once impregnated, Nature, by
mean the informing soul that presides over the whole
which I
organic system, and gives intelligent guidance to every
part, carries forward its development, as nearly as can
be observed, in the following order :
The ovum is, from the first, enveloped in two mem
branes, the outer of which is called the chorion, the
inner the amnion. Within lies the principle of life, the
germ of the complex being. The ova of all the higher
animals are alike at this period, and one can not be dis
tinguished
from another. The amnion, or inner mem
brane, secretes upon its inner surface the liquid in
which the foetus is suspended during the whole period
of gestation. The chorion, or outer on covering, the
other hand, acts outwardly, throwing out villi, which,
gathered at one point, at a certain period, unite with
vessels on the inner surface of the uterus, and form the
placenta,
from the blood of the mother.
or afterbirth, by which the foetus is nourished
This central germinal point, with its two coverings,
form the three parts of a regular cell formation—cell,
nucleus, and nucleolus.
While the ovum is gradually passing down the fallo
pian tube, propelled by the action of its ciliary bodies, a