THE DEVELOPMENT OF THE MOLE. 167 
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 differentiation 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 
