PLACENTAL ANIMALS. 155 



the lowest stage to the very highly developed mind-organ 

 of the M.onkey and of Man. (Cf. Chapter XX.) The 

 human mind is but a more highly developed ape-mind. 



The milk-glands of Placental Animals, as of Marsu- 

 pials, are provided with developed nipples; but the pouch 

 in which the immature young of the latter are carried 

 about and suckled is never present in the former. Nor are 

 the marsupial bones (ossa marsupialia) present in Pla- 

 cental Animals ; these bones, which are embedded in the 

 abdominal wall, and rest on the anterior edge of the pelvis, 

 are common to Pouched Animals and Cloacal Animals, ori- 

 ginating from a partial ossification of the tendons of the 

 inner oblique muscle of the abdomen. It is only in a few 

 beasts of prey that insignificant rudiments of these bones are 

 found. The hook-shaped process of the lower jaw, which 

 characterizes Pouched Animals, is also entirely wanting in 

 Placental Animals. 



The character, however, which especially distinguishes 

 Placental Animals, and which has justly given its name to 

 the entire sub-class, is the development of the placenta, or 

 vascular cake. We have already spoken of this organ, in 

 describing the development of the allantois in the human 

 embryo (vol. i. p. 882). The urinary sac or allantois, that 

 peculiar bladder which grows out of the posterior portion of 

 the intestinal canal, is, we found, formed at an early stage in 

 the human embryo just as in the germs of all other Amnion 

 Animals. (Cf. Figs. 132-135, vol. i. p. 377-380.) The thin wall 

 of this sac consists of the same two layers, or skins, as the 

 wall of the intestine itself; internally of the intestinal-glan- 

 dular layer, and externally of the intestinal-fibrous layer. 

 The cavity of the allantois is filled with fluid ; this primi- 



