THE GENETIC CONTROL OF PATTERN I97 



ture-sensitive period during the first two days after pupation; but this 

 general period can be broken up into a set of shorter periods during 

 which the stimulus affects one particular element of the pattern. The 

 individual parts of the pattern which can be identified in this way can 

 be recognized not only in related species but in quite distant ones; 

 possibly it will be found that comparatively few such elements are 

 responsible for all Lepidopteran patterns. By comparative studies it 

 may be possible to say that a certain element which might be ex- 

 pected is missing in a certain species; for instance, the central part of 

 the mid-field in the upper side of the forewing of Vanessa urticae; and 

 in this case the missing element, which is as it were squeezed out of 

 the normal wing, can be produced by a heat stimulus applied at the 

 appropriate time. The stimulus has apparently made a new element in 

 the pattern, but clearly the potentiality for this element must have 

 been implicit in the genotype of the moth, although normally unex- 

 pressed. 



The genes which have so far been studied all seem to affect the degree 

 of realization of the fundamental patterns which, as we have said, are 

 common to large groups of Lepidoptera. There must be other genes 

 which determine the underlying potentialities for these patterns, but 

 we know very little about them. 



Goldschmidt^ has recently described another case of the control of 

 degree of realization of a pattern. The allelomorphs of the vestigial 

 locus in D. melanogaster produce wings which look as though httie 

 pieces had been cut off from the tips. The study of the development 

 of the various forms shows that this is almost Hterally true. With the 

 less extreme allelomorphs, at least, the wings develop quite normally 

 until after the imaginal discs have been everted, but then undergo a 

 degeneration of parts of the distal ends. Goldschmidt could not decide 

 between two alternative explanations; either some substance necessary 

 for development diffuses into the wing from the proximal end, and in 

 mutant phenotype does not reach the tip, or some substance which 

 causes degeneration is produced in the tip and diffuses from there 

 towards the proximal end. In either case, if one arranges the wings in 

 a series of increasing degeneration, one obtains a picture of the course 

 of the diffusion stream of the hypothetical substance (which would of 

 course proceed in opposite directions according to the two alternadve 

 hypotheses). The various allelomorphs can easily be arranged in a 

 series of increasing effectiveness, and their relative dosage value 

 ^ Goldschmidt 1937, 1938, 



