SEGMENTATION OF THE OVUM OF THE SHEEP. 241 



majority of specimens there is no trace of a metagastrula stage. 

 lu two specimens I find that some cells are distinctly darker than 

 others ; these cells are mostly internally placed, but they come 

 to the surface at more than one place and at nearly opposite 

 poles of the embryo. I have drawn similar sections in my 

 former paper (1, figs. 18, 19). 



So, although I would not now assert that a metagastrula 

 stage does not exist in the rabbit, I must reiterate that I have 

 been unsuccessful in finding satisfactory evidence of it. But if 

 it does exist, I believe it to be open to the same interpretation 

 as that which I have placed upon the similar condition in the 

 sheep. 



In the same way it is not difficult to explain the develop- 

 ment of the mole as described by Heape. In the absence of 

 any very marked difference between the characters of the two 

 groups of cells it is not easy to deny that any of the cells of 

 the inner mass have been derived from the outer wall of cells. 

 In the sheep the characters are markedly different. The 

 doubling of the outer layer at one point is clear. 



In the mole (as in the pig) there is no such marked differ- 

 ence. Hence it is not possible to deny or to affirm such a 

 course of events. 



Heape gives the subsequent separation of the hypoblast in 

 these words : — " Certain of the cells bordering the blasto- 

 dermic cavity become separated off from the main portion of 

 the inner mass, and form a single layer of cells bordering the 

 mass on the inner side. This layer is the hypoblast. The 

 hypoblast is therefore derived from cells which result from the 

 multiplication of the inner cell mass present in the fully 

 segmented ovum.^^ 



But is it not possible that these cells may have originally 

 been derived from the outer layer ? Unfortunately Heape had 

 no real sections of any stage younger than his fig. 17, in 

 which the cavity of the blastodermic vesicle had attained 

 a great size. 



The difference which Heape detected between the " single 

 layer of cubical hyaline segments " surrounding " an inner 



