220 RICHARD ASSHETOr^. 



for the sheep ; and apart from this, there seems to me to be 

 such a strong prima facie case for the hypoblastic blasto- 

 dermic vesicle theory that I believe it to be deserving of more 

 attention than it has received. Hitherto no mammal with a 

 typical blastodermic vesicle stage has been examined which in 

 any way hinted at the explanation which is suggested by the 

 study of the segmentation of the sheep's ovum. 



While my hypothesis is not entirely in accord with that of 

 any former author, it resembles those of Minot (43) and 

 Robinson (46), and indeed owes much to the work of the latter 

 upon the rat and mouse, and to the theoretical conclusions of 

 his suggestive essay in vol. xxxiii of the ' Quarterly Journal 

 of Microscopical Science.' 



The chief feature of my hypothesis is that (like Minot and 

 Robinson) I regard the main wall of the blastodermic vesicle 

 as entodermic. I differ from them, however, in that I regard 

 as entodermic a greater portion of it than they do. 



I differ from Minot in considering the inner layer of the 

 inner mass as entoderm as well as the whole of the subzonal 

 membrane (v. Minot's ' Human Embryology,' pages 107 and 

 108). Hence the cavity is not, as he regards it, the segmenta- 

 tion cavity, but the archenteron, which arises as a split within 

 the entoderm as shown in my diagrams 2 and 3, and is not com- 

 parable to the segmentation cavity of amphibians. 



Haddon's (27) theory, although called by Minot a similar 

 explanation to his, seems, however, to me to be very different. 

 Haddon regards the subzonal epithelium, including " Decken- 

 zellen," as the equivalent of the extra-embryonic epiblast of 

 the bird's egg ; so that the cavity of the blastodermic vesicle, 

 which he compares with the yolk-sac, is supposed to be 

 bounded on all sides except the embryonic pole by " extra- 

 embryonic" epiblast, and not by hypoblast. Keibel's (38) 

 suggestion is practically the same as that of Haddon. With 

 Minot I should agree in so far as supposing that, to quote his 

 words, ** there is, then, a complete inversion of the germinal 

 layers in all placental mammals." This I imagine to occur in 

 most forms at a very early stage (pig, sheep, rabbit, mole. 



