Bauhaus-Universität Weimar

The Cyclopaedia of Anatomy and Physiology, vol. 5: Supplementary Volume
Todd, Robert Bentley
PLACENTA — (Functions). 
Fig. 487. 
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 
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- 
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


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