34 THE DESCENT OF THE PEIMATES 



the other modes of development in some way 

 or other, as special adaptations that have indeed 

 to be explained, but that are of no primary 

 importance. 



The reason why this point has been so little 

 in dispute may partly be ascribed to the force 

 of habit, partly to the consideration that a mode 

 of amnion formation which is prevalent among 

 all birds and reptiles, must in the nature of 

 things be a more ancient and a more primitive 

 mode. All the more this appeared to be the 

 correct view, as the majority of the mammals 

 do conform with Sauropsida in the way in which 

 they form their amnion. 



And so it requires a certain amount of deter- 

 mination to single out the exceptional cases 

 which we encounter amongst a small minority 

 of mammals, and to pretend that in them a 

 more primitive arrangement is preserved. 



I will try to explain to you in a few words 

 which are the chief points at issue in these con- 

 flicting views. Let me first, then, remind you 

 that the amnion is a membrane, continuous with 

 the body wall and stretching hindwards so as to 

 form a closed sac over the embryo's back. In 

 it the young animal is enveloped as in a pro- 

 tective watercushion. Indeed, the amnion is un- 



