588 FORMATION AND USES OF PLACENTA. 
-the whole surface of the chorion, a number of little tufts shoot 
out (fig. 325), which come into contact with the lining mem- 
brane of the uterus; and this is furnished with a multitude 
‘of glandular follicles, which secrete a nutritious fluid that is 
absorbed by the tufts of the chorion, and by them communi- 
ated to the embryo. When the allantois is formed, it serves 
to carry the blood-vessels of the embryo to the inner surface 
of one part of the chorion ; and they shoot through this, so as 
Fig. 325.—IntTer10r oF HumAN Urervs at the seventh week of Pregnancy: 
b, outlet of the uterus, of which the walls c, ¢, c, c, laid open by incision, are turned 
back to display its contents; d d, its lining membrane; g, tufted surface of the 
chorion; g2, its internal aspect; 4, hk, amnion; i, yolk-bag; &, umbilical cord; 
Z, embryo. 
to dip-down, as it were, into large expanded vessels that extend 
outwards from the walls of the uterus. In this manner is 
formed, in all the higher Mammalia, the important organ 
termed the Placenta; which essentially consists of the ramifi- 
cations of the foetal blood-vessels contained in the Umbilical 
Oord or “navel-string,” ensheathed by prolongations of the 
large vessels of the maternal uterus. In the Marsupials and 
Monotremes (Zoowocy, §§ 309—320), no placenta is ever 
formed, the embryo coming into the world in a stage scarcely 
more advanced than that represented in fig. 325. In either 
case, the vessels of the embryo are enabled to absorb from 
the blood contained in those of the parent, through the thin 
