416 Action of the Genetic Material 



into a larger thorax, the anterior half of which is real thorax and the 

 posterior half a mirror image of it, the wing origin of which could 

 never be suspected if found alone. In the process both scutella, the 

 real one and the mirror image from alula anlage, are crowded out 

 and disappear. This is an extreme example of how embryonic 

 regulation can make a completely new whole out of two completely 

 diflFerent structural parts, once the normal alignment and determination 

 has been changed by a genically controlled action upon one focal 

 embryological condition. 



In detail the processes may be of different order, as far as the 

 relative roles of induction, redetermination, and regulation come into 

 play. If we think of the stages in which an appendix of wing type, 

 a pseudo-wing, is formed — anteriorly the histological differentiation 

 (all bristles) being that of a half scutum; posteriorly, of a half 

 scutellum — clearly two things are superimposed upon each other: 

 primary wing determination and secondary thorax determination in 

 the same organ. It might be said that gross morphology keeps some of 

 its original determination, but for stoppage of growth and differenti- 

 ation. The latter are of the newly determined type. This redeter- 

 mination might mean the "victory" of an ever present thorax deter- 

 miner. But it could also mean that the stoppage of wing growth and 

 the eversion of the wing disc in a wrong position with broad attach- 

 ment to the thorax produces a situation of the type which we called 

 a "raw edge," and that embryonic regulation sets in, transforming the 

 available tissue into a mirror image of the adjacent normal part, the 

 thorax. The "redetermination" would then combine features of mosaic 

 and inductive development plus regulatory processes. It presupposes 

 also something which we called "labile determination." All this fits in 

 with a discussion by Stern (1940) of mosaic development within 

 imaginal discs, where he pointed out that the early fixation of pro- 

 spective significance of different discs, as wholes, is not in contradiction 

 wdth a less rigid cell lineage of parts within a disc. 



I refrain from discussing further details, which concern experi- 

 mental embryology more than genetics. For our purposes, the study 

 of integration of genically controlled actions into a unified and orderly 

 development, the main points are obvious. They are the amazing 

 interplay of all the different types of genie action (which we have 

 discussed and which will be enumerated again below) by a rather 

 rigid system of control of quality, timing, direction, threshold condi- 

 tions, segregations and movements of material, over-all effects of 



