222 PHYSIOLOGICAL GENETICS 



study begun by Pearson and Duncker of right-left correlations to 

 determine their eventual genetic causation. There is the large 

 body of facts relating to symmetry and asymmetry in identical 

 twins (Newman, Bonnevie, Wilder, etc.). The innumerable 

 facts, both statistical and experimental, are relevant to the 

 present discussion only in so far as they permit linking the action 

 of the gene with the process of pattern formation. 



Let us begin with certain facts that have been found in the 

 cases that we have already discussed. In the studies upon the 

 scalloping effect in the vestigial wing of Drosophila and similar 

 cases, as well as in many cases of bristle abnormalities in Dro- 

 sophila, the following fact was found by all observers. When 

 the very first steps of scalloping are produced either genetically 

 or by external agencies, i.e., the absence of a few hairs at the wing 

 tip up to a small nick, this happens almost invariably only in one 

 wing. This seems to be a rather universal phenomenon, when- 

 ever a genie effect is not strictly symmetrical. It may be 

 demonstrated for the unsymmetrical types of the mutants of 

 Drosophila, wherever the very low members of a series are 

 known, e.g., scalloped wrings of the beaded and beadex type, wing 

 venation of the plexus, delta, extra-vein type, eye defects of the 

 kidney type. It may be demonstrated also for the different 

 phenocopies. Another typical example is the mosaic-like struc- 

 ture of the wings of male intersexes in Lymantria, which are 

 shown in Fig. 35. 



If we remember now the explanation given for the produc- 

 tion of scalloping, the first beginning of scalloping would be 

 produced by a small insufficiency of some growth' substance or, 

 reciprocally, by a small amount of lytic substance above the 

 threshold, acting late in wing development. If this were a 

 localized action of the mutant gene, both wings ought to react 

 simultaneously, except for a local variation in thresholds or the 

 like. If, however, the substance in question enters the wing 

 from its base, spreading through the wing, and if this substance 

 is produced in common for both wings, the amount distributed 

 to each wing is liable to considerable variation. For example, if 

 95 per cent growth substance is available instead of 100 per cent, 

 one wing may receive 46; the other, 49 per cent. If the threshold 

 for scalloping effect is 48 per cent, only one wing will be scalloped. 

 (In case of interpretation with lytic stuff: five units above the 



