1 86 H. L. WIEMAN. 



The granules may therefore be of the nature of chromatin, 

 and actually represent the chromatin of the nurse cells ; for the 

 chromosomes never appear in these cells after they have divided 

 amitotically. But even though this be the case a nuclear origin 

 for these granules loses any special germinal significance when 

 we remember that the latter develop into yolk as well as pole 

 disc, and are therefore just as much "yolk determinants" as 

 " germ cell determinants." 



It would appear that a fact of far greater importance in the 

 determining of the germ cells lies in the migration of the pole 

 cells to a position outside of the yolk. This migration isolates 

 these cells and places them in an environment that is entirely 

 unique. It is rather difficult to understand how the absorption 

 of pole disc granules could be the cause of this. The fact that the 

 pole disc occupies a position between the pole cells and the yolk 

 gives a considerable foundation for regarding it as a source of 

 nutrition for these cells. 



Hegner states that the granules of the pole disc are absorbed 

 by the pole cells in passing through ; after which no further im- 

 portance is attached to the pole disc. However the granules are 

 not all taken up by the cells in their migration and the greater 

 part of them remains behind after the cells have passed through 

 (Fig. 6). 



If then the pole disc represents a part of the nutritive stream 

 of the ovum that has not been transformed into ordinary yolk, 

 but instead has been reserved to supply the pole cells, the con- 

 clusion presents itself that the latter as a result of this special 

 kind of nutrition undergo a peculiar method of metabolism which 

 differentiates them from the somatic cells. When these cells are 

 ready to immigrate into the embryo through the pole-cell canal 

 the differentiating factor has already acted and the germ cells are 

 readily distinguished by certain morphological peculiarities from 

 the somatic cells. 



It would be highly interesting to know whether or not the 

 germ cells will develop in the absence of the pole disc. The 

 experiments devised by Hegner to test this point have been nega- 

 tive in results, although they do show that the egg may have its 

 contents profoundly disturbed without preventing the production 



