THE EVOLUTION OF MAX. 



that it appears exclusively in the higher Mammals, not in, 

 the lower. Of the three sub-classes or principal groups of 

 the Mammals, the two lower groups, the Beaked Animals 



PI 



FIG. 138. Egg-membranes 

 of the human embryo (diagi-am- 

 matic) : w, the thick, fleshy wall 

 of the uterus ; plu, placenta (of 

 which the inner layer (plv') 

 sends processes in between the 

 tufts of the chorion (cTiz) ; clif, 

 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- 

 bryo, not represented here) ; 

 dg, yelk-duct ; ds, yelk -sac ; 

 dv t dr, decidua (dv, true, dr, 

 false decidua). The cavity of 

 the uterus (uh) opens below 



into the sheath (vagina), above, on the right, into the oviduct (/) (After 



Kolliker.) 



(Orniihostoma) and Pouched Animals (Marsupialia), have no 

 placenta, the allantois remaining a simple bladder, filled 

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

 most highly developed mammalian sub-class, the Placenta! 

 Animals, is a true placenta developed from the allantois. 

 To the placental 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 has developed from this group of Mammals. 



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 



