THE DESCENT OF THE PKIMATES 37 



above-named representatives of four different 

 orders of mammals : Cavia (guinea-pig), Eriiia- 

 ceus (hedgehog), Pteropus (bat, flying-fox), IIo?no 

 (man). This will suffice to convince you that 

 as far as placental mammals go, the statement 

 that the amnion arises as an upward-growing 

 fold, which finally encapsules the embryo, does 

 not find universal application. There is indeed 

 an equal chance of the other mode of formation 

 having been the original one. My own choice 

 is fixed upon the latter hypothesis because in 

 the Amphibia, from which I suppose the earliest 

 placental mammals to have been derived, we 

 find arrangements that appear to explain the 

 earliest origin of the amnion in the way here 

 advocated. There is, moreover, no difficulty in 

 tracing both umbilical and allantoidean pla- 

 centation to a disposition of parts such as we 

 encounter in the Amphibia. It would, however, 

 lead me too far if I should attempt to take you 

 over the whole ground covered by this hypoth- 

 esis, and it is more than time to turn back to 

 the realm of facts. 



The facts to which I wish to call your atten- 

 tion, and which are confirmatory of the views 

 here developed, have been brought to light by 

 different observers, at different times. They all 



