32 WALTER HE APE. 



pore," and compared by him to the blastopore of other Verte- 

 brata. 



The fully-segmented ovum is therefore considered by this 

 author to be comparable to the gastrula stage of other verte- 

 brata. 



I have hitherto been unable to confirm the account given by 

 Beneden of the segmentation, but am by no means therefore 

 disposed to conclude his careful descriptions are inaccurate. 

 At the same time it appears to me obvious, from the subse- 

 quent development of the mole, that his view^s of the homolo- 

 gies both of the two layers of segments and of the " blastopore" 

 are incorrect. 



In the first place, the so-called "entoderm" segments will 

 be found to give rise to the greater part of the epiblast of 

 the embryo ; and in the second place, the structure of the 

 primitive streak will be seen strongly to confirm Balfour's 

 opinions (vide Nos. 1 and 3) of the homology of that organ 

 with the blastopore of lower types. 



Had Beneden's interpretations been correct, however, and 

 had the inner mass really been entodermic, the fully-segmented 

 mammalian ovum could not even then be compared to the 

 gastrula condition of Amphioxus ; for whereas the enteric 

 cavity of the latter is within the entoderm cell mass, that of 

 the former is eventually found to be outside those cells, and 

 between them and the ectoderm. 



Up to this point in the development no differentiation of 

 the segmentation spheres into epiblast and hypoblast has yet 

 taken place, and there is indeed, as I will show later, no 

 evidence of any differentiation until some considerable time 

 after the completion of segmentation. 



The structure of the mammalian ovum at the close of 

 Stage A is therefore seen to be, as far as we can now tell, 

 entirely unlike that of any other animal; and until we have 

 some knowledge of the steps by which mammals were evolved, 

 it appears to me useless to attempt to draw any homologies. 



It may be interesting to note that although the earliest con- 

 ditions of mammalian development cannot be compared with 



