THE MUTATED GENE 77 



interpreted as due to a collaboration of facet-forming and facet- 

 destroying processes. The catalytic action of the Bar gene might 

 then mean an acceleration of the destroying processes. This 

 interpretation, of course, does not change the principle involved. 



It is not possible to record here all cases in which the applica- 

 tion of the rate concept to gene-controlled processes has led to a 

 better understanding. We shall have to discuss some further 

 examples in connection with other problems. Only a few more 

 cases may be mentioned as types of applicability of the concept 

 to various processes different in detail. 



A typical gene-controlled character with a visible action in 

 time is the molting of insects between the different instars. It 

 is known for the silkworm (Tanaka, 1916; Ogura, 1932) and for 

 the gypsy moth (Goldschmidt, 19246, 1933a) that races exist with 

 different numbers of molts (3 to 5 in the silk worm; 4 to 5 in 

 Lymantria). The genetic difference involves a series of multiple 

 allelomorphs controlling the different types. Their effect may 

 be shifted by the presence of modifying genes (Ogura) and also 

 by external conditions like feeding with coal dust (Nagai) 1 

 or hunger (Nagamori; Goldschmidt). 2 From Goldschmidt's 

 work on the growth curves of such races, it is clear that the num- 

 ber of molts is determined independently of growth in general. 

 The last molt, pupation, of course sets an end to further molts, 

 and this is determined by a definite concentration of a hormone 

 produced in the corpora allata near the brain (Wigglesworth). 

 Ogura, who made a detailed genetical analysis of the case in 

 the silkworm, came to the conclusion that all his facts could 

 be harmonized by the assumption that the different allelomorphs 

 control the rate of production of this hormone, which acts when a 

 threshold value is reached. Modifiers and external conditions 

 will then act in the same way as described above for other cases. 



Of a very different type is the following example which shows 

 how the rate concept accounts for complicated morphological 

 changes. In analyzing the corkscrew type of deer horns, 

 Rhumbler (1929) found that the normal growth requires the 

 exact timing of two processes, viz., the rate of growth of soft 

 preosseal connective tissue and the rate of the ossification ascend- 

 ing from below. If the latter process has too low a rate, the 



1 Quoted from Ogura. 



2 Unpublished. 



