36 PHYSIOLOGICAL GENETICS 



two interpretations, proposed by Steiner, fall completely in line 

 with all the other facts discussed in this chapter, viz., a genie 

 action through the regulation of the velocity of one or another 



process. 



In the case of blue forms, the lipochrome is involved only in so 

 far as it is completely absent. If this deficiency is combined 

 with low melanin formation (as in yellows), the result is white. 

 The olive colors, however, depend upon a change in the arrange- 

 ment of the air-filled tubules in the medullar cells; the respective 

 gene has therefore a morphogenetic effect. (Regarding different 

 degrees of lipochromes, Duncker has derived physiological 

 explanations from his genetic work on canaries and buderigars. 

 But his analysis has not been substantiated by chemical or 

 morphogenetic facts.) 



As a rather remarkable case might be mentioned the develop- 

 ment of the mutant aristapedia of Drosophila, according to 

 Balkashina (1929). This mutant belongs to the group of 

 homoeotic changes, where one organ is transformed into a homol- 

 ogous one. In this case, the bristle on the antenna, called 

 arista, is transformed into a tarsus by a simple recessive mutation 

 (Fig. 14). In normal flies, segmentation of the antenna in the 

 imaginal disk begins in a larva of 4 to 4}4 days (25°) and ends in 

 the young pupa. In an aristapedia fly, however, segmentation 

 begins in a 2-day-old larva, when the segmentation of the leg 

 Anlage also takes place. This segmentation is at once of the leg 

 type and continues thus. We shall return later to this remark- 

 able case and point out here only that the immense morpho- 

 genetic effect seems to be produced by nothing but an earlier 

 incidence of a morphogenetic process. 



The cases thus far considered all involve changes in develop- 

 mental rates. The actual processes involved are not clearly 

 indicated, although it is possible to draw inferences about them. 

 There are also cases of a much simpler type, some of which will 

 be reported in the chapter on rate-controlling genes. Here only 

 two of them may be mentioned because they illustrate two 

 simple types of mutant development. Gabritschevsky and 

 Bridges (1928) studied the development of the giant race of 

 Drosophila and found that it is indistinguishable from normal 

 development up to the time at which pupation ought to occur. 

 The giant larva then continues to grow a few more days before it 



