74 J. W. JENKINSiON. 



blastoderm, but certainly not derived from the " Neben- 

 spermakerne '^ of Riickert and Oppel. The function of this 

 structure is simply " Dotter-resorptiou/' and its nuclei do not 

 form any part of any embryonic layer whatever, but swell, 

 divide directly, disorganise, and ultimately disappear; while 

 the hypoblast proper is derived later on from the blastoderm. 

 If this were so, then on the assumption that the mammalian 

 ovum was once megalecithal and similar to that of the Sau- 

 ropsida, the theoretical reasons which Assheton has urged 

 would lose greatly in force ; for it would not in that case be 

 the trophoblast and hypoblast which were derived in the 

 segmented ovum from a cell or group of cells separate to 

 that which gave rise to the epiblast, but the epiblast and 

 hypoblast which arose in common from a group of cells, 

 namely from the inner mass, which was distinct from the 

 trophoblast. 



At the same time it must be remembered that in the Am- 

 phibia yolk-cells and hypoblast are identical, and that in the 

 Gymnophioua we have what appears to be a condition inter- 

 mediate between the former and the Sauropsida; though even 

 here, judging by Brauer's figures (9), a good many yolk- 

 nuclei are left unaccounted for after a definite epithelium has 

 been formed to make the floor of the gut. 



It might still be urged, of course, that the trophoblast was 

 the representative of the " Dottersyncytium," but this is not 

 the point in Assheton's paper. Minot (23), indeed, has insti- 

 tuted a comparison between the Mammalian blastocyst and 

 the segmented ovum of Amphibia, in which he does homo- 

 logise the trophoblast of the former with the yolk-cells of the 

 latter, which he supposes to have grown over the epiblast as 

 Rauber's " Deckschicht" (see figs. I, K). He, however, does 

 not treat the hypothesis at great length, and dismisses rather 

 lightly the somewhat fatal objection that in the mammalian 

 blastocyst hypoblast cells are found at an early stage under- 

 neath the epiblastic mass. 



But, setting aside these perplexing and somewhat unpro- 

 fitable speculations, there is a much more serious objection 



