412 Action of the Genetic Material 



pendently by genie aetion. The normal result requires the interplay 

 of at least two determining stuffs with competition for a common 

 substrate or threshold conditions, with timed processes of growth; or 

 timed and directed gradients of determination during a labile period; 

 or timed ending of the labile period and increasing irreversibility. The 

 mutant might possibly interfere with only one of the integrated 

 processes (e.g., initial growth of the end segment of the anlage) in 

 order to produce an orderly upset of the entire determinative system, 

 which probably contains the typical ingredients of all such systems, 

 determining stuffs, gradients, exact timing, threshold conditions. 



The topic is so important for the theory of genie control of 

 development that another remarkable example will be discussed: the 

 mutants of the podoptera and tetraltera group in Drosophila (Gold- 

 schmidt, Hannah, and Piternick, 1951; Rapoport, 1943; Goldschmidt, 

 1952a,b). Here the mutant or mutants (multiple factors, major 

 mutants, and minor modifiers are at the genetic basis) produce com- 

 plicated structures instead of a wing, leglike appendages, structures 

 resembling a haltere (tetraltera), and transformation of definite parts 

 of the wing anlage into thorax. Again these effects show variable 

 penetrance and expressivity from an almost normal wing through all 

 transitions to the extremest type with no wing and a duplicated thorax. 



It is known that the mesothoracic imaginal disc is determined 

 very early (probably already in the embryo; Geigy, 1931; Henke et ah, 

 1941), so that the genie actions to be analyzed occur before any 

 differentiation is visible. The dorsal mesothoracic disc produces most 

 of the dorsal and lateral walls of the thorax and the wings. The wing 

 part of the disc becomes visible first in the pre-pupa, when it separates 

 from the thoracic part and assumes a kind of segmentation by the 

 appearance of transverse folds. This condition might be compared 

 with that of all other appendages so far as the wing starts as a triseg- 

 mented rudiment and its growth and differentiation into a wing blade 

 occur mostly in the tip segment. 



The series of the expression of mutant action requires the as- 

 sumption (already anticipated by Berlese on grounds of comparative 

 morphology for the insect wing in general) that the wing anlage has 

 four centers of independent determination for, in an anterior-posterior 

 order, (1) the anterior cubital region of the wing (leaving aside a 

 still more anterior part not relevant for the present discussion); (2) 

 and (3) the rest of the wing blade (2 and 3 being the anterior and 

 posterior halves); (4) the alula, which is rather independent also in 

 the finished wing. The decisive action of the pod ( podoptera ) and tet 



