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) is a 

 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 



