54 J. P. HILL. 



area occurs on tlie non-formative hemisphere of the same 

 vesicle. 



These spherical cells are, I am convinced, of no morpho- 

 logical importance, and are destined sooner or later to de- 

 generate. They have certainly nothing to do Avith the 

 entoderm, the parent-cells of that layer arising exclusively 

 from the foi*mative hemisphere and not from cells such as 

 these, which ai'e budded off from both hemispheres. The fact 

 that they are, in unilaminar vesicles, more numerous over the 

 formative hemisphere may perhaps be taken as an indication 

 of the greater mitotic activity of the formative as compared 

 with the non-formative cells. 



The P r i m i t i V e E n t o d e r m a 1 Cells . — Following closely 

 on the stage represented by these '01 blastocysts is the ex- 

 tremely important one constituted by the '99 and '04 vesicles 

 before referred to. This stage is tbe crucial one in primary 

 germ-layer formation, and marks the transition from the uni- 

 laminar to the bilaminar condition, since in it the entodermal 

 cells are not only distinctly recognisable as constituents of the 

 formative region, but are to be seen both in actual process of 

 separation from the latter and as definitely internal cells, fre- 

 quently provided with, and even connected together by, 

 pseudopodial-like pi'ocesses of their cell-bodies. Such cells 

 are already present in the '01 vesicles (fig. 71), and probably 

 also in the blastocysts in which the sutural line first makes 

 its appearance, but are much less conspicuous than in these 

 older blastocysts. 



The '99 blastocysts are distinctly more advanced than the 

 '01 batch and are just a little earlier than the '04 lot. The 

 former measui'ed, as already mentioned, 4'5 mm. in diameter, 

 the latter 4'5 and 5 mm. (the majority being of the latter 

 size). In my notes on the intact '99 vesicles I find it stated 

 that one hemisphere, forming rather less than half of the 

 entire extent of the vesicle wall, appeared somewhat denser 

 than the other, the sutural line marking the division between 

 the two. I naturally inferred at the time that the denser 

 hemisphere corresponded to the embryonal region of the 



