THE MUTATED GENE 239 



In the case considered as 3, there was no determination stream 

 that could have different beds. But the causative agent for the 

 functioning of each center of diffusion (at each bristle) might 

 change in regard to the time of the onset of action (or the thresh- 

 old), and there might be more or less order in this respect; this 

 would give the same phenotypic result as the different types of a 

 determination stream. Sturtevant and Schultz (1931) thought 

 of the type with one point of outlet. Goldschmidt (19316) tried 

 to derive from the facts a system with five such points. Dubinin 

 and Friesen (1932) as well as Child (1936) think, however, that 

 the actual variability of the behavior of the individual bristles 

 does not show the type of correlation that would be required if a 

 determination stream should coordinate the behavior of groups of 

 bristles that it would have to reach in a definite order. This 

 is a potent argument in favor of the possibility mentioned under 3. 

 The recent work of Nujdin (1936), at least in part, supports these 

 conclusions. He made a special study of the correlation of 

 different areas by comparing their behavior in mosaics, exhibiting 

 different areas containing bristle- and hair-affecting genes 

 (scute 8). It is to be remembered that the mesonotum (dorsal 

 thorax and scutellum) is derived from one pair of imaginal disks. 

 The sternopleura might develop from either the dorsal or the 

 ventral mesonotal disk, and the humeri are formed from an 

 independent disk. He found that all areas derived from different 

 disks behave independently. Within the mesonotum he finds 

 nine areas on each side that behave as units in regard to mosaicism. 

 This means that in the differentiation of the mesonotum at some 

 time a subdivision takes place which results in a pattern of the 

 surface of the mesonotum. According to Nuj din's description, 

 some fields in this pattern contain only one, and others two, 

 bristles (nine areas and 1 1 bristles) . This, then, means that most 

 of the bristles, but not all, are found in areas that have been 

 formed as independent parts of a pattern. It has to be added 

 that within a given area a stronger correlation exists in an 

 anterior-posterior direction. This means that the bristle pattern 

 is the result of a formation of areas in a definite pattern, a 

 formation that is bilaterally independent, proceeds generally in 

 an anterior-posterior direction, and takes place almost simul- 

 taneously. If this description is correct, one might conclude 

 that it is not a series of determination streams that are involved 



