338 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



hemispheres, smaller than in the rabbit ; its lateral lobes are far 

 larger than the median one, and the flocculi are insignificant. 



There are no corpora trapezoidea. 



7. Placenta. This is of the type known as metadiscoidal, for, 

 although it is circular like that of the Rabbit, yet, to begin with, 

 villi are developed over the entire surface of the chorion, though 

 they only persist over a limited area. These villi, too, are much 

 branched, and their relation to the maternal tissue is complex. 

 It will also be observed that the placenta is opposite the ventral 

 surface of the embryo, the reverse being the case in the Rabbit 

 (Fig. 95). The yolk-sac (umbilical vesicle), again, is of less 

 importance. The thickened mucous membrane lining the uterus 

 is known as the decidua, because it is shed at birth. That part 

 of it which forms the maternal portion of the placenta is the 

 decidua serotina, that lining the rest of the uterus is the decidua 

 vera, and a decidua reflexa surrounding the embryo is formed by 

 the outgrowth and fusion of folds. 



Comparison of Man with the Highest Apes. 



The apes which come nearest to Man in structure are the 

 Gibbons (Hylobates), Orang-outan (Simla satyrus), Chimpanzee 

 (Troglodytes niger), and Gorilla (T. gorilla}. In none of these is 

 the erect attitude maintained without the assistance of the fore- 

 limbs, the easiest and most habitual position. Consequently 

 there are not the same perfect adaptations to this attitude which 

 are formed in Man, and of which the most striking are the 

 following: (1) Double S-shaped curve of the vertebral column, 

 (2) hind-limbs longer than fore-limbs, (3) feet completely special- 

 ized for terrestrial locomotion, (4) pelvis broad and short. Both 

 pollex and hallux are proportionately much larger in Man, but 

 the latter is not opposable as in apes. 



The dental formula of Man and the higher apes is the same, 

 but in Man the teeth form a continuous curve above and below 

 (there being no diastema), and the canines are not so well 

 developed. The canines of apes are disproportionately large 

 when the jaws are closed their points fit into gaps between the 

 incisors and canines above, between the canines and premolars 

 below. 



The most distinctive anatomical features of Man, besides those 



