EXPERIMENTAL STUDY OF HEREDITY 75 



Weismann expanded the above conception of hereditary 

 substance by calling attention to the fact that it must 

 contain elements, not only from one individual or pair, 

 but from a long line of ancestors. He called the idioplasm 

 (of Nageli) in the germ-cells germ-plasm, and the heredi- 

 tary units, "necessary to the production of a complete 

 individual," he called ids. Each id contains a full com- 

 plement of hereditary factors necessary to produce" a 

 perfect plant or animal. The germ-plasm corresponds 

 to the deeply staining chromatin of the cell-nucleus, and 

 the ids are grouped together in idants, which correspond, 

 in general, to the chromosomes. Weismann further 

 postulated that the ids were composed of "primary con- 

 stitutents," which he called determinants, and that every 

 character independently inherited has its own determinant 

 in the germ-plasm. Finally Weismann postulated that 

 each determinant is a complex of biophors (the ultimate 

 units of matter in the living state), each biophore being 

 composed of (non-living) chemical molecules. Thus we 

 "rise through his categories as follows, from atom to mole- 

 cule, from molecule to biophore, from biophore to deter- 

 minant, from determinant to ids, from ids to idants 

 (chromosomes), which are composed of the hereditary 

 substance or germ-plasm; schematically as follows: 



germ-plasm (chromatin) 

 idant (chromosome) 

 id. 



determinant (factor, of Mendel) 

 biophore (biogen, of Verworn) 

 molecule 

 atom 



The germ-plasm is continuous from generation to genera- 

 tion, and therefore possesses a kind of physical immortality. 



