220 PHYSIOLOGICAL UENETKS 



feather germ. Either a rhythmic production of a growth sub- 

 stance produces in the cambium zone of the feather germ an 

 alternation of quicker and slower cell division with the same 

 result a* in mollis, viz., that the cells that enter their growth 

 phase later are the ones that can produce the pigment; or, within 

 the cambium, a simultaneous "stratification" takes place very 

 early, resembling the Liesegang ring model (rings in test tubes), 

 and determines which cells may form pigment. In spite of much 

 interesting work, the essential problem — the pattern-forming 

 action of the gene — has hardly been attacked in this case. 



Not much more is to be said about the related problem of the 

 agouti hair in mammals in which alternate zones of different 

 pigmentation exist. Dry (1928), who studied this case, ascribed 

 this pattern to an alternation in the hair follicle, conditioning the 

 quantitative aspect of a pigment-forming process in the following 

 sequence: high = black; intermediate = yellow; low = "low- 

 black. " No information is obtained concerning the causation 

 of the rhythm. 



In concluding this section, we may finally refer to work 

 relevant to the problems under discussion but undertaken more 

 from the standpoint of experimental embryology than from a 

 genetical viewpoint. Embryologists frequently have made 

 transplantations of organs between animals of different hereditary 

 size (Harrison, 1924, 1929; Twitty, literature, 1934); and, as a 

 rule, the transplants kept up their own hereditary growth rate, 

 which is to be expected if the determination occurs rather early 

 (see above). But there is also an influence of the host, i.e., 

 the environment visible, which acts upon the transplant by 

 functional, mechanical, or nutritive means. More difficult to 

 explain is the influence that different parts of the organ exercise 

 upon each other: a lens from a larger species working with an 

 eyecup of a smaller species gives a compromise result, each part 

 adapting itself to the different growth rate of the other (Harrison). 

 This belongs, of course, to the difficult subject of regulation. The 

 fact that such regulations usually require contact of the parts 

 indicates at least that principles not different from those involved 

 in ordinary induction may be at work. But there is also a 

 regulation of a different type. If older organs are transplanted 

 into a younger host, or younger ones into an older host, both are 

 influenced so that normal growth relations ultimately result 



