250 BIOLOGY: GENERAL AND MEDICAL 



division; and the combination of ids contained in them, although 

 not a permanent one, persists for some time. 



"The 'germ plasm, or hereditary substance of the metazoa and 

 metaphyta, therefore, consists of a larger or smaller number of 

 idants, which in turn are composed of ids; each id has a definite 

 and special architecture, as it is composed of determinants, each 

 of which plays a perfectly definite part in development. 



"The development of the primary constituents in the germ 

 plasm of the reproductive cell takes place in the course of the cell 

 divisions to which the autogeny of a multicellular organism is due, 

 in which process all the ids behave in 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 autogenetic stages 

 gradually become poorer as regards the diversity of their determin- 

 ants, until they finally contain only a single kind. 



"Each cell in every stage is in all cases controlled by only one 

 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 penetrate 

 through the nuclear membrane into the cell body; and there, 

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

 bring about the histological differentiation of the cell, by multiply- 

 ing more rapidly at the expense of those biophors already forming 

 in the cell body. Each determinant must become 'ripe/ and 

 undergo disintegration into its biophors at a definite time or at a 

 certain stage of autogeny. The rest of the determinants 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 firs^a definite 

 kind, but afterward undergoes continual and systematic changes 

 in consequence of the uneven rate of multiplication and gradual 

 disintegration of the ids. 



"The apparatus for cell-division is only of secondary importance 

 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 cell-divisions 

 cannot, moreover, be determined by the centrosome, although it 

 produces the primary stimulus; the apparatus for division is set 

 in motion by the cell, which is controlled by the idioplasm. Were 

 this not the case, the nuclear matter could not be the hereditary 

 substance, for most of the hereditary characters of a species are due 

 in a less degree to the differentiation of individual cells than to the 

 number and grouping of the cells of which a certain organ or 



