152 EICHARD ASSHETON. 



As I have stated on a previous page, the hypoblast of the 

 embryonic area is a network at first. Also, I believe that 

 at first many of the outermost cells of the extra-embryonic 

 area are really isolated. These will be under less tension than 

 those near the embryonic pole, as they will, if they are connected 

 at all with other hypoblast cells, have connecting strands upon 

 one edge only. Hence these preserve for a longer period their 

 rounded contour. 



The ultimate conversion of the isolated cells into a network 

 (or a series of networks) and of the networks into thin 

 continuous membranes, and of thin continuous membranes into 

 columnar membranes, would seem, therefore, to be but the 

 result of increase of rate of multiplication over rate of 

 expansion. 



The inner layer of epiblast cannot be said to have come into 

 existence as a layer until after the formation of the hypoblast. 

 Until that moment it formed, together with the future hypo- 

 blast, the inner mass. It was impossible, except in as far as 

 could be premised from their position, to say from their 

 characters which would be inner epiblast cells and which hypo- 

 blast cells (v. fig. 28). What I believe takes place is this. Those 

 cells of the lenticular inner mass which, being at its edges, are 

 removed by the expansion of the wall of the vesicle, and those 

 which are in direct connection with the cells so removed, 

 become by virtue of their position the future hypoblast ; the 

 remainder become the inner layer of epiblast. That is to say, 

 those cells of the inner mass which are not influenced by the 

 expansion of the vesicle, as above described, and are accordingly 

 upon that part of the wall, though not actually as yet part of it, 

 which is least affected by the expanding forces, become the 

 inner epiblastic layer. 



Of all the cells, therefore, in the embryo at this time, these 

 (the inner epiblast layer) are least affected by external causes. 

 These cells are the more free to assume their natural shape, 

 which I believe to be spherical, and are only slightly flattened 

 between the two layers, outer epiblast and hypoblast. 



