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 pezedo- 
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 
