30 PHYSIOLOGICAL GENETICS 



certain threshold); and furthermore, thai the time of visible onsel 

 of this gene-controlled deficiency is a simple function within 

 the increasing series of allelomorphs. Without going into further 

 details here, these facts indicate that here, again, processes of 

 definite velocity are controlled by the different mutant genes and, 

 also, processes having velocities of different but typical rate in 

 series of alleles with increasing phenotypical effect. 



It is encouraging that Harnly (1936), whose studies on the 

 sensitive period of the vestigial series have been mentioned, 

 derived from his facts (which have to be restated in different 

 terms, as the development of the vg-wing was not yet known to 

 him) a very similar interpretation. He concludes (1) that there 

 is a definite pattern of wing development in time (in a later 

 chapter, we shall see that it is not pattern of development but 

 pattern of a determination stream); (2) that the degree of expres- 

 sion is dependent upon the duration and rate of processes 

 occurring in the larval period of the vestigial genotype; (3) that 

 mutations at the vestigial locus apparently affect the duration 

 and rate of these developmental processes. 



A considerable amount of work has been done on the eye 

 mutants of Drosophila, though the results are far from being 

 complete. The eyes are formed from an imaginal disk which 

 separates from the pharynx as the so-called cephalic complex 

 after 8 to 16 hr. of larval development (total 206 hr. at 27°C). 

 Between 36 and 48 hr. at 27° this complex is divided into optic 

 and antennal buds, the former now being the eye disk proper. 

 This grows and differentiates until it is finished — except for the 1 

 pigment — at about 84 hr. 



The mutants of eye form have been studied by Chen (1929), 

 Krafka (1924), Johannsen (1924), and others. The most com- 

 plete account thus far available is for the mutants conditioning 

 decreasing size of the eyes glass 2 , eyeless 2 , Lobe c , studied by 

 Medvedew (1935). This author finds that the differences 

 between these types (including the variability and asymmetry 

 of eyeless) are already visible at 24 hr. (27°C), though they are 

 rather small. From this time on, growth proceeds perfectly 

 parallel in regard to percentual increase. Figure 13 shows the 

 curve of growth for these four types, age of larva plotted against 

 logarithms of disk size. The initial differences as well as the 

 parallel growth rate is visible. The drop of the curves at 48 hr. 



