138 RICHARD ASSHETON. 



about 60° from the upper pole iu all directions (fig. 28, 

 HY. I.). 



From this moment the apparent changes for some hours 

 seem to concern the innermost layer of cells of the embryonic 

 disc, and the straggling cells. 



The hypoblast, as a perfectly definite layer, is formed by the 

 time that the blastodermic vesicle measures '5 mm. in diameter, 

 that is, by about the 102nd hour after coition. It is not, how- 

 ever, as yet by any means a continuous membrane, it is a net- 

 work or a fenestrated membrane. For this reason in section 

 it appears to be represented by isolated cells lying beneath the 

 embryonic disc (v. fig. 29, HY.). 



Certain cells can be detected in earlier stages which from 

 their being more lenticular, and more inwardly placed, can 

 probably be described as hypoblast cells (v. fig. 28, HY.). This 

 stage is about the ninety-eighth to one hundredth hour, when 

 the diameter of the blastocyst measures about '36 mm. Also 

 all the apparently — and in many cases I believe actually — 

 isolated cells {HY. I. in figs. 28, 29, 37—40) can from the 

 moment of their apparent wandering be termed hypoblast. 



By the separation of the cells round the periphery of the inner 

 mass, and certain others of the more inwardly placed of the 

 inner mass into what we may call hypoblast, there are left 

 cells in between the hypoblast and outer layer, which do 

 not show the same tendency to be flattened or drawn out as the 

 others. As is well known, this intermediate cell layer forms 

 part of the epiblast, and so may be termed the inner layer of 

 epiblast. To this I shall refer later. 



I wish now to refer to the straggling cells {HY. I.) of the 

 inner mass, and consider why they apparently wander round 

 the inside of the blastodermic vesicle. That they arise from the 

 inner mass I have no doubt. 



I have explained above the flattening of the inner mass as 

 due to being drawn out by the general expansion of the walls 

 of the vesicle, and to this may also be due the original isola- 

 tion of certain cells round the periphery of the inner mass as 

 seen in figs. 28, 37, and 38. 



