340 The Unity of the Organism 



contact Avitli tlie nucleus. The first pigmentation ap])ears 

 in a very thin layer over the whole outer surface of tlie 

 nuclear membrane. From here it increases uniformly all 

 around the nucleus and gradually fills the entire cytoplasmic 

 part of the cell, the nucleus itself, however, remaining free 

 from pigment. The absence of pigment from the nucleus 

 of pigmented cells is, as is well known, of wide occurrence. 

 In this case at least the evidence seems conclusive, as the 

 author says, "that the nucleus plays an essential part in 

 pigment formation".^ What that part may be is the im- 

 portant question, and one as yet by no means fully answered. 



Several investigators hold that the chromatin of the nu- 

 cleus is the direct source of the })igment. Thus, for example, 

 Aurel von Szily reports that in the vertebrate eye the me- 

 lanin granules are produced from the colorless rod-like bod- 

 ies derived directly from the chromatin of the nucleus. These 

 bodies he calls pigment bearers {Pigmenttrdger) }^ They 

 pass out of the nucleus through the nuclear membrane and 

 become disseminated through the cytoplasm, where they are 

 gradually transformed into melanin granules. 



Much more evidence might be brought forward on the 

 moi*phological side that the nucleus at least and probably 

 its chromatin takes a direct part in the production of 

 brown pigment. And the supposition is strengthened and 

 extended by evidence produced in recent years of how, chem- 

 ically speaking, the nucleus does its work. The idea that the 

 nucleus is specifically concerned in the oxidative processes 

 of the cell had been gradually gaining definiteness for sev- 

 eral years before 1902, at which time R. S. Lillie produced 

 apparently conclusive evidence to this effect so far as con- 

 cerns frog tissues. He subjected living active cells to re- 

 agents which indicate oxidation in the animal body by change 

 of color.* 



* Several such reagents are known but the one chiefly relied on by 

 LiUie and which has since been frequently used for the same purpose is 



