242 Chas. W. and G. T. Hargitt, 



Germ Layers 



One can find other sections of gastrulse which appear to show 

 an ingression of cells (fig. 32), but in all our preparations this is 

 due only to orientation, for, if the gastrula be cut only slightly 

 oblique, some cells of the entoderm are cut more or less trans- 

 versely and give the impression of a mass of cells. Only when the 

 plane of the section passes through the long axis of the invagina- 

 tion can the true condition of affairs be found. Whether the cells 

 sometimes .found free in the cleavage cavity do actually take part 

 in entoderm formation may possibly be answered by the conditions 

 show in fig. 34. Here is a condition similar, perhaps, to those 

 found by Hyde where an immigrated cell was joining the in- 

 vaginated cells. It is questionable, however, whether this cell is 

 actuallytakingpartinthe entoderm formation forthe invagination 

 in this embryo is nearly completed and the entoderm cells 

 arranged in a compact row except for the indentation caused 

 by the cell %' . A similar cell, 'a\ is present in the coelenteron; 

 this together with the position of '6,' and particularly the con- 

 dition of the layer of entoderm cells near this cell, suggest the 

 probability that cell '6' is being forced, or is migrating, through the 

 entoderm layer, into the coelenteron. Such cells in the coelenteron 

 degenerate. Further evidence touching the fate of the cells free 

 in the cleavage cavity is found in the condition of their nucleus. 

 In agreement with Smith (1891), Hyde (1894), and Hein (1900) 

 it was found that in Aurelia these cells rarely (in our preparations, 

 as in Smiths's, never) showed an intact nucleus, but there were 

 only deeply staining granules present which may have been bits of 

 a fragmented or degenerating nucleus. The presence of only scat- 

 tered chromatin grains in the cells is pretty good evidence of the 

 degeneration of those cells as Smith and Hein pointed out, though 

 Goette (1900) and Hyde (1894) believe it does not warrant this 

 interpretation. The entire absence of any sign of nucleus or even 

 of chromatin fragments, which is true of some of these cells, 

 suggests either a further degeneration of the cells or, more proba- 

 bly, that they are only cytoplasmic masses pinched off from some 

 cell and hence doomed to degeneration. It is possible that in 



