THE GERM-PLASM THEORY 293 



cell, becomes at once active and breaks up into 

 smaller and smaller groups 'of determinants corre- 

 sponding to the building up of the body, while the 

 germ-plasm in the other remains in a more or less 

 'bound' or 'set' condition, and is only active to the 

 extent of gradually stamping as germ cells the cells 

 which arise from the primordial germ cell." 



According to Weismann this actually occurs in 

 Dipterous insects, but there is no evidence in the 

 literature to warrant this statement. It is conse- 

 quently necessary to imagine the germ-plasm as 

 present but not definitely localized in a germ cell 

 until some time after the two-cell stage has been 

 reached. Thus in hydroids Weismann explains the 

 situation as follows: "Here the primordial germ 

 cell is separated from the ovum by a long series of 

 cell-generations, and the sole possibility of explaining 

 the presence of germ-plasm in this primordial germ 

 cell is to be found in the assumption that in the 

 divisions of the ovum the whole of the germ-plasm 

 originally contained in it was not broken up into 

 determinant groups, but that a part, perhaps the 

 greater part, was handed on in a latent state from 

 cell to cell, till sooner or later it reached a cell which 

 it stamped as the primordial germ cell." 



Evidence that the germ-plasm does become sooner 

 or later localized in the primordial germ cell has accu- 

 mulated rapidly within recent years. In the psedo- 

 genetic fly, Miastor (see Chapter III), the first cell 

 to be cut off from the egg is the primordial germ cell 

 (Fig. 17, p.g.c.), although at this time there are 



