150 KIOHARD ASSHETON. 



epiblast layer. But when the embryonic disc is broken up in 

 such a way as to scatter and tear apart the various cells, then 

 there undoubtedly appear to be strands passing from some of 

 these rounded cells to others of the same layer. At the same 

 time I cannot say positively whether these strands are really 

 connections between the cells in question, or whether they are 

 fragments and shreds derived from the tearing apart of the 

 hypoblast or outer epiblast layer, between which they are 

 placed. But however this may be, the cells of the inner epi- 

 blast layer are, at the time I am speaking of, either isolated or 

 else connected only at certain spots of small area. These are 

 of the rounded type. 



At the outer limit of the hypoblast there are also cells, some 

 of which, I believe, may be quite isolated; others are connected 

 to each other by a few strands of protoplasm. These approach 

 very closely to the rounded type of cell. This is the type of 

 cell which I believe to be the most natural, by which I mean 

 the least influenced by its environment. This is the type of 

 cell which first comes into existence in the segmentation of the 

 ovum, when, within the protecting investments, the cells, at first 

 uninfluenced by pressure or tension from without, or from each 

 other, assume their natural or spherical contour. As the seg- 

 mentation proceeds, the inner segments become the more com- 

 pressed, and assume polygonal forms. 



After the establishment of the cavity of the blastodermic 

 vesicle, the outer cells, by the pressure from within increasing 

 more rapidly than their rate of multiplication, are drawn out 

 into thin plate-like cells. 



These outer layer cells are from the first connected with each 

 other by their edges, and form a continuous membrane, a 

 condition without which in all probability the formation and 

 enlargement of the blastodermic vesicle could not be produced. 

 As long as this tension within is maintained at a rate greater 

 than the rate of multiplication of the cells, the cells retain 

 their flattened condition. 



What of the inner mass cells ? Upon the removal of the 

 pressure of the outer layer, the more outwardly placed of the 



