THE MUTATED GENE 191 



the tip last; or that the diffusing substance is a lytic substance 

 which is first accumulated in a quantity above the threshold of 

 action in the wing tip with all the other graded consequences. 

 The action of many other genes is of a similar type; the truncate 

 wing and the miniature wing in Drosophila are examples in 

 which other substances are involved, in the former case acting 

 only upon mesoderm differentiation; in the miniature case, only 

 upon growth after differentiation. This type of growth sub- 

 stances, in insufficiency and in hyperproduction as well as 

 eventual lytic substance, might be responsible for a large majority 

 of developmental processes in mutants, especially wherever 

 linear growth or reduction is involved. They might also be 

 responsible for all processes of differential growth; but here, of 

 course, the pattern factor will come into play, perhaps in the form 

 of different resistance to the diffusion of the substance in differ- 

 ent directions of space. This part of the problem will be treated 

 in a later chapter. It must be emphasized that in all these cases 

 the unity upon which the mutant gene acts is a complicated 

 system. The unit affected might be the whole organism (size 

 and proportions) or a whole organ (an imaginal disk, a wing) 

 or a definite part of an organ (the wing veins as a whole or one 

 definite vein). Within these units, therefore, no mosaic forma- 

 tions are possible except when the last stages of development are 

 involved (see miniature wing, Demerec, page 176). Moreover, 

 different processes of this type, conditioned by different mutant 

 genes, controlling the production of different substances, might 

 be combined in the same individual without interfering much, 

 with each other. For example, Goldschmidt (1937) showed that 

 the large broad wing of the mutant expanded in Drosophila is 

 determined even in the stage of the imaginal disk. A combina- 

 tion of the mutant genes expanded, dumpy, cut can produce a 

 wing that is short-dumpy, broad and cut, in addition, because 

 three substances are involved, produced at different times and 

 acting upon different tissues, viz., upon the general growth and 

 upon epithelial and mesodermal differentiation (for data see 

 Csik, 1933; Goldschmidt, 1937). If the same reactions with the 

 same end product were involved, a compromise result, e.g., 

 intermediate, would be encountered. This, of course, is the case 

 with compounds of multiple allelomorphs; but it might also be 

 the case with different <rciics. e.g., different genes for scalloping, 



