.-.I 



PHYSIOLOGICAL E N E TK 'S 



controlling color or shape or :my other character and a gene con- 

 trolling growth rate. This is true. But as it seems that all 

 gene actions may be in the end reduced to the control of rates, 

 such cases where actually visible rates are involved are of a special 

 significance. Only a few typical examples may be given, which 

 illustrate the type of processes involved. There are some cases 

 in which the mutant gene changes the rate of differentiation 

 without any visible changes of growth. Thus the vestigial gene 



450 



400 



350 

 I 300 



D. 

 O 



J 250 



£ 

 *S 200 



E 150 



100 



50 - 



\tt 



15 20 25 

 Length in mm. 



Fig. 17. — Graph of average number of melanophores in the three types of 

 goldfish, Common TT, heterozygous TT l , and transparent T l T l . (After Good- 

 rich and Hansen, 1931, J. Exp. Zool. 59, Fig. 7.) 



retards development during the larval period by 5} ^ hr. according 

 to Harnly (1929). More typical are the cases in which the 

 mutant gene controls a different rate of growth processes. 

 Studies of such cases have revealed a few facts of primary impor- 

 tance. Castle and Gregory (1929) showed that the differences 

 between large and small races of rabbits, which genetically are 

 caused by multiple genes, are, in fact, differences of rate of cell 

 division, which begin early in development. Thus the growth 



