THE MUTATED GENE 



197 



parts or fields. One such characteristic field (see Fig. 36) is 

 the symmetry field in the center of the wing, limited on each 

 side by a system of bands. Its unity is proved by the fact that 

 it acts as a whole when changed in phenocopic experiments and 

 that it is affected as a whole by mutant genes. Definite tempera- 

 ture treatments may broaden this field if applied at a sensitive 

 period or else narrow it. The same action is produced by a 

 mutant gene Sy, which constricts this field. To find out how the 

 size of this field is controlled, Kuehn and Henke (1936) (see also 

 Kuehn and Engelhardt, 1933) destroyed definite parts of the 



Fig. 36. — Diagrammatic representation of the elements of the pattern in 

 nymphalids. Interpretation: left, by Schwanwitsch; right, by Sueffert. (From 

 Henke, 1935, Verh. deutch. Zool. Ges., Fig. 15.) 



developing wing at different times and watched the effect with 

 the following result: If the wound is made at the proper time 

 (early pupa), the bands limiting the symmetry field are pushed 

 around the wound in a definite way, which is well illustrated in 

 Fig. 37. The resulting pattern is such as would be caused if a 

 determination stream proceeded from the anterior and posterior 

 margin of the wing in a broad front toward the center and if the 

 edge of this stream, whever it comes to a standstill, determined the 

 position of the bands limiting the symmetry field. A comparison 

 of Fig. 37 with the diagram (Fig. 38) beautifully illustrates the 

 actuality of this conception. It is clear that earlier operations 

 will produce this edge effect nearer to the wing margin and later 

 ones toward the center and that still later ones might only affect 

 the contour of the bands which become transverse after the two 

 streams have united (see Fig. 37). Here, then, the embryological 



