THE GERM-PLASM THEORY 295 



evident just before maturation ; it has increased in 

 amount to approximately sixty-four times its former 

 mass. How this increase has been brought about is 

 not known, but it has been suggested (p. 68) that 

 preexisting particles of pole-plasm may grow and 

 divide, or the dilution of the pole-plasm caused by 

 the growth of the egg might start into action some 

 catalyst which would cause the production of more 

 substance like the pole-plasm and cease its activity 

 when the amount of pole-plasm characteristic of 

 the mature egg had accumulated and brought it to a 

 state of equilibrium. In the midge, Chironomus, 

 the primordial germ cell is segregated even earlier 

 than in Miastor, namely, at the four-cell stage. 

 The later history of the germ cells is not so well 

 known in this species, however, as in Miastor. 

 The data presented in Chapters V and VI prove 

 that a definite and early segregation of germ cells is 

 known in a sufficient number of groups to indicate 

 that the process is quite general among animals. 

 The morphological continuity of the germ cells, 

 however, cannot be established with such a degree of 

 certainty in the vertebrates, and although most 

 investigators believe that the germ cells are con- 

 tinuous, still the entire keimbahn has never been 

 traced as accurately as it has in many invertebrates. 

 Fortunately almost every new investigation contains 

 additional data and more refined methods which lead 

 us to hope that some time in the near future the 

 primordial germ cells even here may be traced back 

 to early cleavage stages. 



