THE GERM PLASM THEORY ^2U5 



evident just before nialuration ; it lias increased in 

 amount to approximately sixty-four limes its iornicr 

 mass. How this increase lias been brou^'lit about is 

 not known, but it has been su^^f^^esied (p. G8) that 

 preexisting particles of ])()lc-plasni may grow and 

 divide, or the dilution of the j)ole-plasm caused l»y 

 the growth of the egg might start into action some 

 catalyst which would cause the production of niort* 

 substance Hke the pole-plasm and cease its activity 

 when the amount of ])()le-plasm characteristic of 

 the mature egg had accumulated and brought it to a 

 state of equilibrium. In the midge, C/iirunomu.s, 

 the primordial germ cell is segregated even earlier 

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

 The later historv of the germ cells is not so well 

 known in this species, however, as in Minstor. 

 The data presented in Chapters V and \'I ))i()\e 

 that a definite and early segregation of germ ceils 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, 

 how^ever, cannot be established with such a degree of 

 certainty in the vertebrates, and although mu.^L 

 investigators believe that the gvr\^^ cells wvv con- 

 tinuous, still the entire keind)ahn 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 tini(^ in the nivir future the 

 primordial germ cells cvcmi here may be traced back 

 to early cleavage stages. 



