THE DESCENT OF THE PRIMATES 33 



no place is left for an intermediate saurian link, 

 but according to which the placental mammals 

 are connected directly with unknown amphibian 

 ancestors, leaving the Monotremes, the Marsu- 

 pials, and the Sauropsida to come in for lateral 

 connections — as yet wholly unknown — with 

 that more archaic and direct line of descent. 

 This hypothesis is essentially based upon ob- 

 servations that have of late been accumulating 

 concerning the actual mode of development of 

 the amnion in different orders of placental 

 mammals. The development of the amnion in 

 mammals is often said to conform with that 

 which we observe in the chick. This may hold 

 good for the sheep, for the rabbit, and for many 

 other placental mammals, but it certainly does 

 not for man, for the flying fox (Pteropus), for 

 the hedgehog, or for the guinea-pig. In all these 

 representatives of four different orders of mam- 

 mals, the origin of the amnion totally differs 

 from the traditional process just alluded to, 

 although the latter mode is nevertheless met 

 with in other members of those same four orders. 

 Up to the present time embryologists have been 

 in the habit of looking upon the traditional 

 process present in the chick as the typical one 

 from which it ought to be possible to derive 



3 



