620 GEORGE T. HARGITT 



Where in the stages described so far are the germ cells devel- 

 oping? It is impossible to call any of these cells, whether of 

 ectoderm or of entoderm, primordial germ cells, for the reason 

 that they are all so similar in structure and appearance, and 

 most of them so nearly alike in form and size as to render any 

 distinction impossible. All the cells of these buds have the 

 appearance of rapidly growing, undifferentiated cells, that is, 

 all are like embryonic cells. The same appearance is found in 

 the vegetatively developing buds of hydroids, and indeed the 

 growing zones of many animals have the same general appear- 

 ance as those in these developing medusa buds. In these buds, 

 even in stages much older than that shown in figure 48, some of 

 the entoderm cells are so like the ectoderm cells as to make a 

 distinction difficult or impossible. The boundaries of the tis- 

 sues are clearly represented in figure 48 for the sake of distinct- 

 ness, while in reality it is only with oil immersion lenses that 

 the boundary lines can be certainly determined. In figure 47 

 it is uncertain whether the group of cells at the lower part of 

 the bud represent ectoderm cells wholly or whether some of 

 them belong to the entoderm; there is a boundary on both sides 

 of the group of cells, and the shape, size, and appearance of the 

 cells does not sharply differentiate them. 



In a stage considerably later than those described, when the 

 bud has assumed something of the outline of the medusa, and 

 when the stomach is well formed and the canals are laid down 

 in a general way, a section through the stomach wall has the 

 appearance of figure 49. The entoderm cells (to the right in 

 the figure) are beginning to show the vacuolations and granular 

 inclusions characteristic of gastrovascular tissues. Gland cells 

 are also forming. These cells are assuming their definitive form, 

 and the presence of gland cells shows they are ready to assume 

 their definitive functions, if they are not already functional. 

 The outer layer of the stomach, the ectoderm, in which gonads 

 will eventually develop, is still composed of the embryonic cells. 

 Over the greater portion of the stomach the ectoderm is a single 

 layer thick, but about the outer end there is a mass of cells; this 

 end of the stomach is still increasing in size. 



