240 PHYSIOLOGICAL GENETICS 



bul a more or less simultaneous stratification of the whole field 

 into areas followed by the bristle-forming reaction. In a general 

 way, these results agree with those of Rokizky (1930). 



The question is what this would mean for the problem of 

 bristle pattern. This pattern means the formation or not- 

 i'ormation of one (or two) bristles within certain areas. (It 

 might also be a destruction of bristles after their formation, as 

 suspected but not yet proved.) Child has found that all indi- 

 vidual bristles in a given mutant line have the same temperature- 

 effective period. This indicates that the bristle pattern is not 

 concerned with the moment of bristle formation, as expected from 

 the foregoing considerations. But there is an important item in 

 Child's last paper: in different mutants of the series there is a 

 difference in regard to the sensitive periods and to the time of 

 development. 



This leads to the last part of the problem. Goldschmidt had 

 assumed that a determination stream is involved, but he was 

 careful to point out simultaneously that this assumption is not a 

 necessary part of the argument, which holds also in case of inde- 

 pendent determination of each bristle. The decisive point was to 

 explain the steplike arrangement of the phenotypic effects in the 

 series. This would be completely explained if the different 

 areas of the pattern in the mutants had a different time of 

 differentiation, the beginning as well as the duration of which is 

 controlled by the different alleles. Expressed in terms of 

 determining substances, this would mean that the outlets would 

 open at different times in the different areas or that the velocity 

 of differentiation became different in the areas or a similar process 

 involving a time element. Such a system, as worked out by 

 Goldschmidt in a model that might be changed in many directions 

 (see also Friesen and Dubinin who constructed a similar model in 

 order to criticize it), accounts sufficiently for all the facts in a 

 general way. The observation of Child, which was just men- 

 tioned, is highly in favor of such an explanation by relative rates 

 of some processes. We refrain, however, from going into more 

 details. If Child's (rather improbable) claim that the step order 

 of the Russian authors and also of Sturtevant-Schultz is of no 

 value were justified, any discussion would be superfluous. Future 

 studies will be necessary to clarify the problem. 



