Chromatin as ^^ Hereditary Substance'' 341 



Although some investigators report having failed to get 

 the differential reaction described by Lillie, on the whole his 

 evidence with much more of like })urport that might be cited, 

 makes the conclusion seem unescapable that for a consider- 

 able range of animals the "nucleus plays an essential part in 

 pigment formation by some activity wliich greatly resembles 

 an oxidizing- action." 



How far tlie cliromosomes are responsible for this activity, 

 is by no means settled. Hooker could find nothing similar 

 to von Szily's "pigment bearers", or evidence of any kind 

 that the melanin granules come from chromatin. Indeed, he 

 brings forward a number of weighty considerations against 

 the theory that in the frog at least the chromatin is directly 

 concerned in pigment production. He holds that his obser- 

 vations demonstrate that in this animal "melanin is formed 

 in the cytoplasm of the cell at the point of known greatest 

 efficiency of the nucleus as an oxidizing agent." 



Summarizing our examination of the direct evidence fa- 

 vorable to the theory of chromosomes, or at least chromatin, 

 as the mechanism of heredity, we fl7id that in the origin and 

 growth of flagella and pigment in some organisms the theory 

 receives a certain amount of support. 



B. INDIRECT EVIDENCE 



The indirect evidence favorable to the theory will now be 

 considered. Significantly enough the theory is supported 

 chiefly by this sort of evidence. To such an extent is this 

 true, and so sterling in quality and great in quantity is the 



a mixture of one of the naphthols with a derivative of one of the ben- 

 zenes. This mixture produces a deep violet-colored fluid on oxidation. 

 By treating kidney tissue, for example, under 2:)roper conditions with 

 this indicator, Lillie found that the "nucleus of the tubule cells remains 

 comparatively clear and uncolored, and that the coloration of the cyto- 

 plasm is diffuse, but typicalli/ deeper in the immediate neiyhborliood of 

 the nucleus than elseichere — a clear indication that oxidations are espe- 

 cially active at the nuclear surface." 



