58 J. r. HILL. 



whole are most abundant in the '04 series, and they also 

 exhibit a remarkable range of variation in shape. They may 

 have a perfectly distinct oval or rounded outline (fig's. 67, 71, 

 72), or, as is more frequently the case, they may lack a 

 determinate form and appear quite like amoeboid cells owing 

 to their possession of cytoplasmic processes of mai-kedly 

 pseudopodial-like character (fig. 69). Frequently, indeed, 

 the cells are connected together by the anastomosing of these 

 processes, so that we have formed in this way the beginnings 

 at least, of a cellular reticulum (figs. 68, 69, 70). 



The question now arises. How do these primitive ento- 

 dermal cells originate from the small, darkly staining cells of 

 the unilaminar formative region designated in the foregoing 

 as the entodermal mother-cells ? I can find no evidence that 

 the primitive entodermal cells are formed by the division of 

 the mother-cells in planes tangential to the surface; on the 

 contrary, all the evidence shows that we have to do here with 

 an actual inward migration of the mother-cells, with or Avith- 

 out previous mitotic division, such inward migration being 

 the outcome of the assumption by the mother-cells, or their 

 division products, of amoeboid properties ; in other Avords, the 

 evidence shows that the formation of the entoderm is effected 

 here not by simple delamination (using that term in the sense in 

 Avhich it was originally employed by Lankester), but by a pro- 

 cess involving the inward migration, with or without previous 

 division, of certain cells (entodermal mother-cells) of the uni- 

 laminar parent layer, a process comparable with that found in 

 certain Invertebrates (Hydroids) and distinguished by Metsch- 

 nikoff as "gemischte Delamination." 



In this connection it has to be remembered that the cells of 

 the unilaminar wall of the blastocyst are under considerable 

 hydrostatic pressure, and, in correlation therewith, tend to 

 be tangentially flattened, though the flattening in this stage 

 is much less than in the earlier blastocysts. From a series of 

 measurements made from an "04 vesicle, I find that over the 

 formative region the ratio of the breadth to the thickness of 

 the cells varies from 6 : 1 to 2 : 1, and even to 3 : 2. On the 



