498 CHARLES H. SWIFT 



either side merge into the flatter cells of the general coelomic 

 epithelium. Scattered among the germinal epithelial cells an 

 ocasional primordial germ-cell is to be seen. In observing these 

 cells the question naturally arises as to whether they have arisen 

 in situ or have reached that position through a migration after 

 having originated elsewhere. Waldeyer ('70), it will be remem- 

 bered, formulated, the former idea. The researches of Hoffman 

 ('93), Nussbaum ('01), Rubaschkin ('07), and others have cast 

 rather serious doubts upon the correctness of this theory. In 

 fact it may be said to have been disproven as regards some of 

 the germ-cells. The fact that typical germ-cells are found in the 

 root of the mesentery and in the coelomic epithelium before the 

 differentiation of a portion of the coelomic epithelium into ger- 

 minal epithelium certainly is proof enough that some of them at 

 least do not arise there. However, the origin of some of the 

 germ-cells from the germinal epithelium has never been posi- 

 tively dispproven. Semon ('87) in supporting Waldeyer's ('70) 

 idea, claims to have seen transitions between cells of the germi- 

 nal epithelium and primordial germ-cells. In that case, in these 

 chick embryos fixed after an incubation of four and one-half days, 

 one would expect to see certain of the epithelial cells with the 

 large attraction-sphere which is so constant and characteristic 

 of the primordial germ-cells. Also, it would naturally be ex- 

 pected that some of them should possess yolk granules, and the 

 large vesicular nucleus and typical chromatin arrangement, while 

 yet possessing the moderate size and cuboidal shape, which is 

 t;>T3ical of the epithelial cells. On the contrary, however, there 

 seem to be no cells present which could in any way be classed 

 as intermediate between the germinal epithelial cells and the 

 true primordial germ-cells. They are all either germ-cells or 

 epithelial cells, and, as far as the four and one-half day chick 

 is concerned, one is forced to the conclusion, either that the 

 germ-cells in the germinal epithelium do not arise in situ but 

 reach that position through tissue growth or migration, or, if any 

 have arisen there through a transformation, it occurred at an ear- 

 lier stage. All recent investigation seems to favor the former 

 alternative. 



