NATURE OF THE PLACENTA. 245 
and remarkable organ, which plays an exceedingly im- 
portant part in nourishing the young one developing in the 
maternal body. The placenta (also called after-birth) isa 
soft, spongy, red body, which differs very much in form and 
size, but which consists for the most part of an intricate 
network of veins and blood vessels. Its importance lies in 
the exchange of substance between the nutritive blood of 
the maternal womb, or uterus, and the body of the germ, 
or embryo. (See vol. i. p. 298). This very important organ 
is developed neither in marsupials nor in beaked animals. 
But placental animals are also distinguished from these two 
sub-classes by many other peculiarities, thus more especially 
by the absence of marsupial bones, by the higher develop- 
ment of the internal sexual organs, and by the more perfect 
development of the brain, especially of the so-called callous 
body or beam (corpus callosum), which, as the intermediate 
commissure, or transverse bridge, connects the two hemi- 
spheres of the large brain with each other. Placental ani- 
mals also do not possess the peculiar hooked process of the 
lower jaw which characterizes Marsupials. The following 
classification (p. 246) of the most important characteristics 
of the three sub-classes will best explain how Marsupials, in 
these anatomical respects, stand midway between Cloacal 
and Placental animals. 
Placental animals are more variously differentiated and 
perfected, and this, moreover, in a far higher degree, than 
Marsupials, and they have, on this account, long since been 
arranged into a number of orders, differing principally in 
the formation of the jaws and feet. But what is even of 
more importance than these, is the different development of 
the placenta, and the manner of its connection with the 
