508 STANLEY C. BALL 



(figs. 22 and 23). Just as Bresslau finds in Mesostomum so in 

 P. gemellipara the ectoderm cells stretch backward in a sheet 

 over the underlying mass (fig. 21). Figure 22, drawn from a 

 section so thick that only the upper third of the embryo was 

 removed, represents the condition just before the close of gas- 

 trulation. The five yolk cells, three of which show the large 

 nuclei and thin cytoplasmic sheets of the secondary entoderm 

 cells whose food they have just become, are being folded into 

 the embryo by the overgrowth of the ectoderm cells. At the 

 right three of the latter are of such a character that they resemble 

 the yolk cells just described. 



It is apparent from the above account that gastrulation in 

 P. gemellipara is brought about by a process of epiboly or over- 

 growth of the entoderm by the ectoderm cells. The latter 

 are formed by differentiation of the external portion of the mes- 

 ectodermic mass, whereby the cells, first at the anterior end, 

 then gradually those lying more and more laterally, become 

 flattened and push backward over the entoderm. 



At this point another phase in the nourishing of the embryo 

 is observed. It was noted above that, after the gorging of the 

 primary entoderm cells, more or less yolk remained in the capsule 

 (figs. 18 and 21), The ectoderm cells, now undertaking the 

 role of nutrition, absorb this free yolk. In figure 22 the cells 

 on the right still show the yolk granules distinctly, indicating 

 that the process here is similar to that carried on by the entoderm. 

 Very soon, however, the yolk undergoes a change. The flakes 

 and granules in each cell flow together into a single lai'ge globule. 

 It is evident from this figure that the posterior cells take in most 

 of the yolk, those at the anterior end coming in contact with 

 only a very little. The nuclei of the ectoderm are characterised 

 by the presence of large and distinct nucleoli. 



It is strikingly evident that in embryos of corresponding stages 

 of development,. particularly that shown in figure 24, the ecto- 

 derm yolk-content of some is decidedly less than of others. The 

 conclusion is that the primary entoderm absorbed the greater 

 part. According to this hypothesis but little yolk would have 



