28 



physiological a i:\etics 



already been destroyed Figs. LO to 14; Goldschmidt, 1935c, 1937; 

 A.uerbach, 1036). In the highest types like vestigial, the 

 process must begin in the young imaginal disk. This, however, 

 cannot be demonstrated but has to be extrapolated from the rest 

 of the scries. 



Similar results have been obtained for a number of other wing 

 mutants like dumpy, cut, pointed, miniature, and beadex, all of 



• - 



I: 



'•■■ ■-' 



■ ■ 



Wf ! ' 



Jftil 



Fig. 10. — Three stages from postpupal development of a Drosophila wing 

 heterozygous for vg/vg n °. At the time of pupation the wing is already incom- 

 plete. (From Goldschmidt, 1937, Univ. of Calif. Publ. Zool., pi. 16.) 



which show normal development up to a certain point, followed 

 by an onset of degeneration of tissue at typical points (different 

 in different cases) or by a slowing up of certain growth phe- 

 nomena of the already differentiated wing. There are, however, 

 also cases where obviously a different growth rate is present from 

 the beginning (mutant expanded). 



These facts, then, show that the mutant gene makes something 

 necessary for normal differentiation to be absent or below the 

 necessary threshold after a certain time in development (or, vice 

 versa, produces something acting as a lytic substance above a 



