230 PHYSIOLOGICAL GENETICS 



is produced l>y a stream oj a substance entering the wing from 

 its base and spreading in a radial direction without fixed beds but 

 not completely without order. Also, in this case, there was a 

 relation between the quantitative features of this flow and the 

 resulting pattern, lather the time allotted to the spreading 

 of the substance determines how much of the wing .surface 

 will be covered, £.< ., the relative amount of male and female area 

 or the amount of substance available is responsible for this. Ii 

 both cases, then, we have a rather irregular pattern effect of 

 different arrangement in detail, controlled by the action of genes 

 The genes in question do not, however, control the pattern 

 directly but reactions of definite speed, leading to production of 

 definite substances at definite times or in definite quantities 

 The diffusion of these substances into the organ — generally 

 described as a determination stream — in beds prescribed by the 

 si ructure of the organ, together with the time of onset or quantity 

 of the flow or both, produces in one way or another the resulting 

 pattern. There can be no doubt that this simplest method is the 

 most frequent one in general processes of formation of embryonic 

 patterns. An indication of this will be found in the fact, also 

 emphasized by Timofeeff (1934), that most of the mutants of 

 morphological characters in Drosophila tend to show r the type 

 of asymmetrical behavior just described for the vestigial case. 

 Color mutants, however, the genes for w r hich do not act through 

 induction of pattern formation, are symmetrical in effect. To 

 this should be added, as a nice check, the fact that vermilion eyes, 

 which we know (see page 186) to be formed partly by an inductive 

 process of the horraonic type, are frequently found in asymmetric 

 expression. 



The same material may lead one step further. We have stated 

 that a series of facts similar to those concerning the vestigial 

 wing of Drosophila was found for the truncated wing (see Gold- 

 schmidt, 1937). In this case, the degeneration of tissue as a 

 result of insufficiency of some substance takes place below the 

 wing epidermis, which remains intact. The w r ing margin is then 

 retracted beyond this degenerating area (Fig. 42), caves in, and 

 forms the truncate wing, which also is known in a series of 

 increasing conditions, caused genetically as well as by phenocopic 

 effect. Here, now, we are facing a perfectly regular pattern, 

 viz., the dumpy shape with curved margin of the Drosophila wing 



