THE DEVELOPMENT OF THE MOLE. 211 



contain larger granules and more granules than they hitherto 

 have done ; and I would suggest that the yolk material ori- 

 ginally contained in all the segments alike, has been trans- 

 mitted from those occupying the outermost layer to those lying 

 within, in order to allow the former segments to perform the 

 function, and exhibit such activity as is now required of them. 



In order to make my meaning clear I will briefly state what 

 these changes are; for a detailed account of this subject, how- 

 ever, I must refer the reader to a former paper (No. 13). 

 Very shortly after the segmented ovum enters the uterus it 

 dilates into a vesicle — the " blastodermic vesicle." In the 

 early stages of this formation the change is due entirely to the 

 activity of the outer layer of segments ; first by a flattening 

 out, and secondly by the multiplication of these cells; the 

 inner mass meanwhile remaining passively attached to one 

 point on the circumference of the vesicle. 



Later the cells of the inner mass assist in the formation of 

 the vesical wall, and eventually the whole of the inner mass, 

 with the exception of a very small number of cells which form 

 hypoblast, become so disposed. The outer layer of segments 

 and the largest portion of the inner mass of segments, there- 

 fore, together form the epiblast of the blastodermic vesicle. 



Eventually the epiblast of the embryo is formed from a 

 portion of the wall of the vesicle, the hypoblast of the embryo 

 from a small number of the inner mass-segments, while the 

 mesoblast has its origin from both epiblast and hypoblast 

 layers. 



Primarily, therefore, the blastodermic vesicle is formed by 

 the energy of the outer layer of segments, and I would suggest 

 that the difi"erentiation of the outer and inner segments, the 

 one from the other, after the ovum enters the uterus, is due to 

 the transmission of yolk contained in the outer segments to 

 the inner segments, this transmission being performed in order 

 that the changes about to take place in the constitution of the 

 ovum may more readily be performed. 



Van Beneden, in his description of the Rabbit's ovum in 

 1875 (No. 5) describes the first two segments formed as the 



