182 Nature of the Genetic Material 



be based upon still larger spectral sections, for example, the yellow- 

 green and, finally, some upon white light, that is, the entire spectrum. 

 In this simile we compare the single wave length to the mutant 

 locus; a few wave lengths, to our chromosomal sections of allelism; 

 the other light effects, to still larger sections of the chromosome; 

 and, finally, white light represents the whole chromosome. The simile 

 would be still more impressive if it were worked out further for 

 chemical effects, in this case, photochemical ones. But, together with 

 the diagrams, it may help to illustrate the concept of a hierarchy of 

 fields of action along the chromosome. ( See the quotation from Mazia, 

 I 2 B a.) 



If the idea is correct, genetic actions should be found which 

 imply that, apart from the specific local action involved in the 

 definition and recognition of a mutant locus, an action of a larger 

 section of a chromosome must be assumed. The following facts 

 (which can be duplicated) may be given as an example of what 

 I mean. We know that a number of so-called homoeotic mutants in 

 Drosophila are located within a rather small section of the third 

 chromosome of Drosophila: polycomb ca. 45.0, proboscipedia 47.7, 

 tetraptera 51.3, aristopedia 48.5, bithorax 58.8, ss-suppressor 63.7, 

 pointed wing 94.1. Thus a series of point mutants is accumulated in 

 a section of a chromosome (with other loci between) all of which 

 affect the determination of segmental appendages (and, in part, 

 segmental determination itself). This may mean that the segment of 

 the third chromosome as a whole (a field) is vitally concerned with 

 the processes of segmental determination. These processes can be 

 interfered with by changes at different points within the field (the 

 mutants just mentioned) or outside the field (the 2d chromosome 

 mutants tetraltera 42.5 and ophthalmopedia 45.0, obviously members 

 of another field ) . The loci within the section, the field, if not mutated, 

 do not individually control the development of the respective imaginal 

 discs so as to prevent them from sidestepping, while the mutant 

 locus produces the sidestep. Rather, the whole intact section controls 

 certain parts of the processes of normal development of the discs; 

 but a disturbance of this action at individual points inside this field 

 or inside another similar field ( in the 2d chromosome ) leads to upsets 

 of development of the discs, all of the homoeotic type but individually 

 different, actually much more different than is the case in the 

 "pseudoallelic" disturbances within a segment described previously. 



We should expect to find more such facts in support of the 

 general idea of the hierarchy of fields. A considerable number of 



