THE Mi TATED OENE 23 



represents the result of this experiment. The ordinate indicates 

 the degree of scalloping (classes I to V) ; the abscissa, the days of 

 hatching; and the dotted curve, the control. On the third day 

 appear the flies thai have been treated with heat, and the addition 

 effect becomes visible. Such additional effecl is, of course, to be 

 expected only in rather simple cases, it- occurrence depending 

 upon the detail- of the processes involved. Kuehn and Henke 

 (1930) state that in the alteration studied by them, i.e., in the 

 distance apart of the bands on the wing of the Hour moth, no 

 additive effect was found. 



2. THE PHENOCOPIES AND THE DEVELOPMENT OF MUTANTS 



Near the bottom of page 3 are pointed out the limitations 

 set to possible changes in developmental processes by the 

 necessity of their integration into a viable whole. These 

 limitations apply both to mutant changes and to phenocopic 

 changes. We saw thai these different causes produce the same 

 end effect, presumably by changing the same developmental 

 processes in an identical way. To understand, then, the type of 

 action of the mutated gene upon development we must be 

 acquainted with the differences between normal and mutant 

 development. 'Hie knowledge of this combined with the facts 

 already reported ought to give a first insight into our problem. 



A. Facts of Mi iwi Development 



To test these or other possible views it becomes imperative to 

 know the details of development that distinguish the mutant 

 types from the wild type. Studies of this type have been called 

 by Haecker phenogenetics, a term that is not accessary but is 

 sometime- useful. Quite a number of facts are known of various 



importance for our problem. 



A first group include- cases that allow only rather general con- 

 clusions. We mention the work of Pernitsch I 1913) and Schnak- 

 enbek (1921), who studied the development of dark and albinotic 

 axolotls. They found that both contain melanophores and 

 xanthophores and that in the albinotic form these pigment cells 

 have a lower rate of growth and multiplication. Goodrich 1 1927 

 found in the fish Oryzias a somewhat different behavior: In 

 different color varieties (mutants), the same pigment cells 

 received different quantities of pigment. Such cases, as far as 



