the germ-cell cycle m 



that in the Cladocera one or more nurse cells act as keimbahn- 

 determinants. The writer (25) several years ago suggested two 

 views as to the probable significance of these inclusions ; first, 

 they may actually represent the idioplasm, and second, they may 

 consist simply of nutritive materials. Even now it is impossible 

 to state definitely the role they play in the germ-cell cycle. We 

 do know, however, that they are peculiar to the germ cells of a 

 number of species belonging to groups that are diverse and 

 widely separated in the animal series, that they are definitely 

 localised in the egg and often in a certain blastomere (the stem 

 cell), and that they disappear some time after the primordial 

 germ cells have been segregated. 



Most zoologists recognise the distinction between germ-plasm 

 and somato-plasm and consider the germ-plasm to be situated 

 within the germ cells. According to this view the primordial 

 germ cell must contain all of the germ-plasm of the developing 

 organism. The keimbahn-determinants are especially important, 

 since they are in some way associated with the germ-plasm 

 before this substance is segregated in the primordial germ cell. 

 It thus becomes possible to determine the position of the germ- 

 plasm in the stem cell and in the undivided egg, or, in the case 

 of many insects, during the early cleavage stages before the 

 blastoderm is formed. One fact that seems more and more 

 certain as the results of investigations accumulate is that the 

 cytoplasm cannot be ignored when the physical basis of inherit- 

 ance is under consideration. My morphological and experimental 

 investigations of chrysomelid beetles seems to prove that the 

 nuclei of the developing eggs are all alike until the blastoderm 

 is formed and that differentiation is controlled by the cytoplasm, 

 the germ cells being produced at the posterior end where the 

 pole-disc granules are situated. Similarly in copepods, Cladocera, 

 and Sagitta, a substance definitely localised in the egg and stem 

 cells and recognisable because of the presence of keimbahn- 

 determinants seems to determine the genesis of the primordial 

 germ cell. Boveri's experiments with Ascaris likewise furnish 

 valuable data which indicate that the nucleus does not initiate 

 the process which results in the formation of the primordial 

 germ cell, but that chromatin-diminution is likewise controlled 

 by the cytoplasm. 



