WEISMANN'S THEORY 135 



it can assimilate food, grow, multiply, but cannot 

 undergo any qualitative change. It makes up the 

 cytoplasm of the cell. The idioplasm, on the other 

 hand, is the main component of the protoplasm; it 

 is the "hereditary substance" and it determines all \ 

 the properties which differentiate certain cells from 

 other cells. 



To this postulation, however, Weismann adds one 

 important detail which constitutes an advance upon 

 Naegeli's views. He endeavours to make his hypoth- 

 esis agree with the actual facts as to structures re- 

 vealed by the microscope. This is why he locates his 

 "hereditary substance" in the nucleus and more par- 

 ticularly in the chromatine, the substance which be- 

 comes visible at the time of cell cleavage and collects 

 then in the form of chromatic granules known as 

 chromosomes, producing various karyokinetic figures. 

 The hereditary substance exists in all the organic cells ; 

 in germ cells, it presents a special composition and is 

 designated by the name of germ plasm. 



The complex make-up of the cells' nucleus reveals 

 an extremely complex hereditary constitution. The 

 nucleus of a sexual cell consists of a certain number 

 of particles which Weismann calls "ids." In many 

 cases ids might be synonymous with chromosomes, at 

 least in all the cases in which the chromosomes are 

 simple, that is, are not composed of several similarly 

 formed structures. In the salt water crustacean (Ar- 



