THE GERM PLASM TIIKORV 293 



cell, bec'oiiu's al oner acllxc and breaks up into 

 sinallor and smaller ^r()U])s oi" (Ictcrnniiaiits corre- 

 sponding to the building up of llic Ixxly, wliilc tlir 

 germ-plasm in the other ri'iuains in a more or less 

 'bonnd' or 'set' condition, and is only actJNc 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 Weismanu explains the 

 situation as follows: "Here the j)rimordial 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 thi> i)rimor(lial germ 

 cell is to be found in the assumi)lion that in the 

 divisions of the ovum th(* whole oi' the gerni-pla>m 

 originally contained in it was not broken up into 

 determinant groups, but that a i)art, i)erhaps 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-])lasm does become sooner 

 or later localized in the i)rimor(lial germ cell has accu- 

 i mulated rai)idly within recent years. In the pa^lo- 



genetic fly, Miastor (see (1iai)ter III), the fir>t (vU 

 to be cut oflF from the egg is the i)rimordial germ cell 

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



