PLACENTA — (Functions).
Fig. 487.
721
The formation of the fœtal chamber more advanced than in fig. 486. The ovum is still evenly covered by villi,
the ends of which, about this time, begin to be attached to the walls of the chamber in which the ovum is
contained.
o, ovum; dr, decidua reflexa, formiug the foetal chamber; dv, decidua rera; u, uterine walls.
ing the formation of a subcutaneous abscess ;
while in proportion as the base of the chamber
becomes extended by the gradual retiring, from
the centre, of the line of reflexion of the de¬
cidua, like waves receding from a central point,
so, at the same time, an increasing surface is
produced by the expansion of the uterus itself ;
and the layer of decidua here formed, com¬
monly termed the decidua serotina, is simply
the mucous membrane reproduced to supply
the place of that which had been consumed
or pushed off in forming the decidua reflexa.
The fourth or final stage consists in the
partitioning of the permanent placenta into
smaller portions or lobes by the extension of
the layer of decidua (serotina) which lies op¬
posite to the developed villi inwards at vari¬
ous points towards the chorion. In this way
are constructed the dissepiments already de-
Supp.
scribed as bounding the several lobes or co>
tyledons. This partitioning of the placenta
commences about the fourth month.
Thus, during these several stages in the
formation of the placenta, two processes may
be said to be concurrently carried on which
tend in opposite directions — a process of
positive enlargement and growth combined
with one of relative retrogression or limita¬
tion. For while the bulk of the placenta is
progressively increasing up to the completion
of pregnancy, the relative amount of surface
of the ovum appropriated to it is, on the other
hand, diminished. The entire surface of the
chorion being, in the first stages of develop¬
ment; employed as a placenta ; while in the
latter half of gestation, one third of it suffices
for that purpose.
Functions of the placenta. — By means of
3 A