218 



PHYSIOLOGICAL GENETICS 



proved by statistical study of the hybrids between shape races. 

 (As a matter of fact, Lindstrom (1928) had already distinguished 

 between genes for size and shape in the tomato.) A study of 

 development showed, then, that the underlying genes actually 

 control the value of k in the heterogonic growth formula, the 

 coefficient of growth partition. This is well demonstrated in 

 Kaiser's (1935) work on Capsicum, where after flowering time, 



fruit shape changes according to the 

 heterogonic growth formula with a 

 definite value of k. A similar case 

 is found in the tomato, where, how- 

 ever, according to Houghtaling 

 (1935), the heterogonic growth is 

 confined to the early development 

 before flowering. In the squash, this 

 period occurs still earlier (Sinnott and 

 Kaiser, 1934). 



At this point, the work of F. R. 

 Fig. 40.— Diagrammatic trans- Lillie and his school on the bird's 



verse section of the base of the f eather ought to be mentioned, 



feather germ. D, dorsal limit; V, . 



ventral limit. Initially barbs are though it is as much Concerned with 



laid down from D to i; later, at pigment pattern as with growth pat- 



v. They are carried by tangential r ° r 



growth from v to u. s, shaft , pri- tern (Lillie and Juhn, 1932; Juhn, 



mordium increasing in the direc- F au lkner, and GustafsOIl, 1931; Juhll 

 tion of the arrows. (P rom Fraps . 



and Juhn, 1936, Phys. Zooi. 9, Fig. and Fraps, 1934). Also, in feather 

 8 -) development, a growth by cell divi- 



sion is followed by enlargement of cells. The growth by cell 

 division takes place, however, not after the fashion of a typical 

 growing animal organ but more after the fashion of growth from 

 a cambium zone in plants. This zone is found in the circular 

 collar of the feather follicle, from which at one dorsal point the 

 shaft is growing out while the outgrowing barbs are lined up 

 around the collar. The growing barbs are constantly shifted by a 

 circular growth process toward the shaft and unite with it, when 

 they have reached a certain length. The new supply of bar!) 

 material is simultaneously pushed up from the ventral side of the 

 collar ridge. Figure 40 of a transverse section through the collar 

 and Fig. 39 showing one-half of the feather germ spread out will 

 help in understanding these relations. Thus, loci of simul- 

 taneous age of the barbs are situated at different points on the 



