184 THE PHILOSOPHY OF BIOLOGY 



The id is supposed to contain all the potentialities 

 of the completely developed organism. It is com- 

 posed of a definite number of determinants, each of 

 the latter being a " factor " for some definite, material 

 constituent of the adult body. There would be a 

 determinant for each kind of cell in the retina of the 

 eye, one for the lens, one for the cornea (or rather for 

 each kind of tissue in the latter), one for each kind of 

 pigment in the choroid and iris, and so on ; every 

 particular kind of tissue in the body would be repre- 

 sented by a determinant. Thus packed away in a 

 particle which lies just on the limits of microscopic 

 vision are representatives of all those parts of the 

 body which are chemically and physically individual- 

 ised, each of these hypothetical " factors ' being a 

 very complex assemblage of chemical atoms. In 

 development the determinants become separated from 

 each other, so that whatever parts of the body are 

 formed by the first two blastomeres are represented 

 by determinants which are contained in those cells, 

 and which are sifted out from each other and segre- 

 gated. As development proceeds this process of 

 sifting becomes finer and finer, until when the rudi- 

 ments of each kind of tissue have been laid down a 

 cell contains only one kind of determinant. This con- 

 sists of biophors of a special kind, and the latter then 

 migrate out from the chromatin into the cytoplasm 

 of the cells in which they are contained, and proceed 

 to build up the particular kind of tissue required. 



The nucleus of the germ-cell is thus a mixture of 

 incredible complexity, but in addition to this material 

 mixture there must exist in it the means for the 

 arrangement of the determinants in the positions 

 relative to each other occupied by the adult organs 

 and tissues. A mechanism of unimaginable complexity 



