THE LAYERS OF THE BLASTODERM. 37 



of the pellucid area, broader, and are more and more filled 

 with white yolk sphericles, till at the line of junction it is quite 

 impossible to say whether a particular cell is a white-yolk cell 

 (sphere) or a hypoblast cell. The white-yolk cells near the 

 line of junction can frequently be seen to possess nuclei. At 

 first the hypoblast appears, to end abruptly against the white 

 yolk ; this state of things, however, soon ends, and there super- 

 venes a complete and unbroken continuity between the hypo- 

 blast and the white yolk. 



Of the mode of increase of the epiblast I have but little 

 to say. The cells undoubtedly increase entirely by division, 

 and the new material is most probably derived directly from 

 the white yolk. 



Up to the sixth hour the cells of the upper layer retain 

 their early regular hexagonal pattern, but by the twelfth hour 

 they have generally entirely lost this, and are irregularly shaped 

 and very angular. The cells over the centre of the pellucid 

 area remain the smallest up to the twenty-fifth hour or later, 

 while those over the rest of the pellucid area are uniformly 

 larger. 



In the hypoblast the cells under the primitive groove, and 

 on each side as far as the fold which marks off the exterior 

 limit of the proto-vertebrae, are at the eighteenth hour consider- 

 ably smaller than any other cells of this layer. 



In all the embryos between the eighteenth and twenty-third 

 hour which I have examined for the purpose, I have found 

 that at about two-thirds of the distance from the anterior end 

 of the pellucid area, and just external to the side fold, there 

 is a small space on each side in which the cells are considerably 

 larger than anywhere else in the hypoblast. These larger 

 cells, moreover, contain a greater number of highly refractive 

 spherules than any other cells. It is not easy to understand 

 why growth should have been less rapid here than elsewhere, 

 as the position does not seem to correspond to any feature 

 in the embryo. In some specimens the hypoblast cells at 

 the extreme edge of the pellucid area are smaller than the 

 cells immediately internal to them. At about the twenty- third 

 hour these cells begin rapidly to lose the refractive spherules 

 they contained in the earlier stages of incubation, and come 



