72 J. W. JENKINSON. 



and at those of Heape, Hubrecht, Selenka, Duval, and others, 

 will show how impossible it is. 



Now, although Asshetou refused to accept these specula- 

 tions of Robinson, he fully acquiesced in the account given 

 by the latter of the development of the mouse ; and having 

 found in the sheep (although not in the pig) what he believed 

 to be evidence for the hypoblastic nature of the trophoblast, 

 he has proceeded to apply this as a generalisation to every 

 other mammalian blastocyst. The only criticism of this is, 

 as before, to refer the reader to the original descriptions and 

 figures of the various authors, an inspection of which will 

 show whether the ingenious diagrams which Assheton has 

 published are defensible or not. It will be more profitable 

 to consider the basis in fact upon which he believed his 

 theory to rest, and the speculative arguments which he has 

 urged in its support. 



Assheton found that in some, though not all of his speci- 

 mens of the segmented ovum of the sheep, certain of the 

 cells which were placed internally stained less deeply than 

 the others (fig. L). These less deeply stained cells he 

 thought he could trace to the epiblastic knob of the blasto- 

 cyst, the outer darker cells to the trophoblast ; and since he 

 found that some of the cells of the inner mass prior to the 

 appearance of the blastocystic cavity (fig. M), and the 

 hypoblast cells in a later stage (fig. N) agreed with the 

 trophoblast, and not with the epiblast in their staining 

 character, he came to the conclusion that the trophoblast 

 was hypoblastic in origin, and formulated the theory that 

 the blastocyst of the placental Mammalia represents a Sau- 

 ropsidau ovum which has lost its yolk, and into which the 

 embryonic epiblast has sunk. 



Now I think it must be admitted that morphological con- 

 clusions drawn from a slight difference in staining reaction 

 are open to grave suspicion, especially when the difference 

 is not to be detected in all the preparations, and is absolutely 

 unsupported by any other evidence, such, for instance, as 

 the direction of the nuclear spindles, which in Assheton's 



