384 



THE EVOLUTION OF MAN. 



that it appears exclusively in the higher Mammals, not in 

 the lower. Of the three sub-cla.sses or piincipal groups of 

 the ManniiaLs, the two lower groups, the Beaked Animals 



Fig. 138. — Ef^fr-membrancs 

 of the human embryo (diagram- 

 matic) : w, the thick, fleshy wall 

 of the uterus; plu, })laccnta (of 

 which the iuncr layer (x>h(') 

 sends processes in between the 

 tufts of the chorion (chz) ; chf, 

 tufted chorion ; chl, smooth cho- 

 rion ; a, amnion ; ah, amnion- 

 cavity ; as, amnion-sheath of 

 the navel-cord (which passes 

 below into the navel of the em- 

 br3-o, not represented here) ; 

 d<j, yelk-duct; ds, yclk-sac ; 

 dii, dr, dfeeidua (dr, true, dr, 

 false decidua). The cavity of 

 the uterus {uh) opens below 



into the sheath (raijina), above, on the right, into the oviduct (^). (After 



Kolliker.) 



(Ornithostoma) and Pouched Animals {MarsiiiyiaU(i), have no 

 l)lacenta, the allantois remaining a simple bladder, filled 

 Avith fluid, as in Birds and Reptiles. Only in the third and 

 most highly developed manunalian sub-class, the Placental 

 Animals, is a true placenta developed from the allantois. 

 To the pUicental sub-class belong the Hoofed Animals, 

 Whales, Beasts of Prey, Insect-eating Animals, Rodents, 

 Bats, Apes, and Men. This circumstance is direct evidence 

 that man lias developed from this grouji of Mannnals. 



In connection with the line of descent of the human 

 race, the allantois is, therefore, of twofold interest : firstly, 

 because this appendage is entirely wanting in the lower 

 classes of Vertebrates, and is developed only in the three 



