THE LAYERS OF THE BLASTODERM. 39 



in size towards the edge of the pellucid area. Here they are 

 about r^th of an inch in diameter, irregular in shape and rather 

 angular. 



The outlines of the cells of the epiblast at this time are 

 easily distinguished from the cells of the hypoblast by being 

 more elongated and angular; they are further distinguished 

 by the presence of numerous small oval cells, frequently at the 

 meeting point of several cells, at other times at points along the 

 lines of junction of two cells (PI. I, fig. 12). These small cells 

 look very like the smaller stomata of endothelial membranes, 

 but are shewn to be cells by possessing a nucleus. There is 

 considerable variation in size in the cells in different parts of the 

 epiblast. Between the front lobes of the brain the cells are very 

 small, 4oVo tn mcn > rising to ^^th on eacn s ^ e - They are about 

 the latter size over the greater part of the embryo. But over 

 the sinus rhomboidalis they fall again to from ^nnjth to 4oVo tn 

 inch. This is probably to be explained by the growth of the 

 medullary fold at this point, which pushes back the primitive 

 groove. At the sides of the head the cells are larger than any- 

 where else in the epiblast, being here about j(^th inch in 

 diameter. I at present see no explanation of this fact. At the 

 periphery of the pellucid area and over the vascular area the 

 cells are T^th to ^^th inch in diameter, but at the periphery 

 of the opaque area they are smaller again, being about the 

 ^oWth of an inch. This smaller size at the periphery of the 

 area opaca is remarkable, since in the earlier stages the most 

 peripheral epiblast cells were the largest. It, perhaps, implies 

 that more rapid growth is at this time taking place in that part 

 of the epiblast which is spreading over the yolk sac. 



