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 
preéxisting 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 Mvastor, 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. 
