454 THE GERM-PLASM 



in the germ-plasm of the reproductive cell, takes place in 

 the course of the cell-divisions to which the ontogeny of a 

 multicellular organism is due, in which process all the ids 

 behave in an exactly similar manner. In the first cell-division 

 every id divides into two halves, each of which contains only 

 half the entire number of determinants ; and this process of 

 disintegration is repeated at every subsequent cell-division, so 

 that the ids of the following ontogenetic stages gradually become 

 poorer as regards the diversity of their determinants, until they 

 finally contain only a single kind. 



Each cell in every stage is in all cases controlled by only 07ie 

 kind of determinant, but several of the same kind may be con- 

 tained in the id ; and the ' control ' of the cell is effected by the 

 disintegration of the determinants into biophors, which pene- 

 trate through the nuclear membrane into the cell-body ; and 

 there, according to definite forces and laws of which we are 

 ignorant, bring about the histological differentiation of the cell, 

 by multiplying more rapidly at the expense of those biophors 

 already forming the cell-body. Each determinant must become 

 '■ ripe,' and undergo disintegration into its biophors, at a definite 

 time or at a certain stage of ontogeny. The rest of the deter- 

 minants in the id of a cell, which are destined for subsequent 

 stages, remain intact, and have therefore no effect on the control 

 of the cell ; but the mode of their arrangement in the id, and the 

 special rate of multiplication of each kind, determine the nature 

 of the next nuclear division — that is, as to which determinants 

 are to be distributed to one daughter-cell, and which to the 

 other. The histological nature of these two cells, as well as the 

 control of their successors, is determined by this division ; and 

 thus the distribution of the primary constituents contained in the 

 germ-plasm is effected by the architecture of the id, which is at 

 first of a definite kind, but afterwards undergoes continual and 

 systematic changes in consequence of the un^eveft^rate bf multi- 

 plication and gradual disintegration of the ids. 



The apparatus for cell-division is only of secondary impor- 

 tance in the process ; its chief part, the ' centrosome,"' like the 

 hereditary substance, is derived from the parental germ-cell or 

 cells, but only constitutes the mechanism for the division of the 

 nucleus and cell, and contains no ' primary constituents.' The 

 rate of the cell-divisions cannot, moreover, be determined by 

 the centrosome, although it produces the required stimulus : the 



