Riddle, Experiments on Melanin Color Formation etc. 313 



It is notable that here for the first time, the student of develop- 

 ment is presented with a "representative particle" in such a form 

 as would enable him to recognize it if he should meet it : Into the 

 open light of day this one comes as a definitely announced chemical 

 individual — chromogen or enzyme ! Everywhere eise in represen- 

 tative particle theories of inheritance, each of the thousands, or 

 millions, of particles — pangenes, biophors, units — has been 

 permitted by its author to parade the whole stretch of the deve- 

 lopmental highway without even so much as a hint as to its name 

 or nature. Indeed, all other theories have asked us to look into 

 the black-encompassed highway at equally black and unillumi- 

 nated somethings ofno known morphological, physiological, 

 or chemical characteristic. 



All now recognize that each and every developmental event 

 as it presents itself to the morphologist, comes as a bündle already 

 wrapped in chemical history. Cuenot proposed to uncover 

 and to open the closed book that deals with the intimate history 

 of a character, and dared even to point specifically to its lines, 

 its words. With this in mind one is better prepared to realize how 

 new the theory, how bold was Cuenot's stand ; and, indeed 

 I am quite willing to say, how warranted by facts then at hand 

 was his attempt to take the dark secret of the development of 

 a character from the embrace of mystery, and give it thus trans- 

 formed and clarified into the- clean palms of chemistry. It was, 

 and is, a beautiful theory, and built upon such encouraging and 

 hopeful lines that many of us must more than half wish that it 

 could be proved entirely true. But how have its assumptions 

 fared at the hands of later investigation ? 



The story is not long, and its several parts may here be briefly 

 sketched. We preferring at this time to present the story with 

 our own experiments as a part of it, rather than attempt a complete 

 and technical description of the latter. The brevity of the story 

 arises from the fact that we are concerned only with the postu- 

 lation of specific chromogens or enzymes as the basis of color 

 formation, and from the fact that this postulate has been followed 

 up with but few of the necessary chemical tests. This, too, in 

 spite of the fact that the presence-absence hypothesis has since 

 been widely accepted by workers in Mendelian lines, and ap- 

 partly by many others as well. The evidence that has been sought 

 in breeding experiments, naturally does not here concern us, 

 since these can furnish at best but very indirect and non-crucial 

 evidence as to the presence or absence of definite chemical Com- 

 pounds. 



It is chiefly in Bateson's laboratory that consistent 

 work has been done directed to the examination of C u e n o t 's 

 fundamental assumptions. But there at least the whole matter 

 has been boldly and vigorously studied. In the Cambridge labora- 



