36 THE DESCENT OF THE PRIMATES 



relation of the embryo either to a big yolk or 

 to a hard shell. 



Most of the hypothetical explanations hitherto 

 proposed have, however, moved upon that basis, 

 with what success we have heard Minot affirm. 

 They do not tell us how the amnion can pos- 

 sibly have developed phylogenetically. In the 

 traditional cases we see it arise as a fold, which 

 slowly and gradually encloses the embryo. And 

 only ivhen the closure has become final is the 

 amnion effective. What, then, were here the in- 

 cipient stages? 



All this appears in another light when we 

 trust to the exceptional cases above mentioned 

 to guide us in determining the phylogeny of the 

 amnion. We can then start from the much more 

 reasonable basis that the amnion at its earliest 

 appearance m%tst have heen a closed sac. Only 

 on this supposition can it be understood that it 

 was of high selective significance from the very 

 first. It was modified only gradually, — one of 

 the modifications being this, that it did not any 

 longer arise so very early in individual develop- 

 ment, but only later by means of the folds 

 alluded to. Now let me emphasize that at the 

 present day we do find this very same develop- 

 ment of the amnion as a closed sac in the 



