186 GERM-CELL DETERMINANTS [CH. 



by possessing chromatin which is absent from the others, 

 but in this case the nutritive cells do not simply extrude 

 chromatin which is lost, but all of it goes to the germ-cell 

 by means of what BOVERI calls a more economical type of 

 differential division. 



The last example to be mentioned is that of the differen- 

 tiation of the spermatogonia from the Sertoli cells in Man, 

 described by MONTGOMERY (191 1 b). In the later generations 

 of spermatogonia a rod-like body is present in the cytoplasm, 

 and in the last division but one this rod passes over into one 

 daughter-cell. Both the cells so produced divide again, and 

 again the rod passes into one of the cells so formed. Of the 

 four cells produced by these two divisions, three are thus 

 without the rod and one contains it. The three cells without 

 it develop into primary spermatocytes and give rise to sper- 

 matozoa, while the cell that contains the rod grows into a 

 Sertoli cell. This case thus differs from all the others men- 

 tioned in the fact that the true germ-cells lack a body which 

 is present in a sister cell that does not become a germ-cell, 

 while in the other cases of differential division the visible 

 differentiating body has marked off the germ-cell from the 

 cells which perform other functions. (Text-fig. 22.) 



In conclusion, the chief impression that is gained from 

 the comparative study of "germ-cell determinants," at least 

 when they are present at the beginning of development and 

 become confined to the cells of the germ-track by differential 

 division, is that although these bodies are evidently strictly 

 correlated with the germ-cells, there is no absolute certainty 

 that they are the cause of the differentiation of germ-cells 

 from body-cells. In Miastor, for example, the germ-cells 

 do not undergo the chromatin-diminution that is character- 



