Genie Control of Development 371 



kinetics of action, thresholds, quantities of end products, and so on 

 accounts for the orderHness of the data. 



The work deals with the mutant vestigial of Drosophila (Gold- 

 schmidt, 1937; Goldschmidt and Honer, 1937 ) , of which a large series 

 of multiple alleles exist. The vestigial effect consists in reducing the 

 wing substance by more and more scalloping in the different alleles 

 until finally it is a small stump. In this series the reduced wing is 

 not miniature but looks as if pieces had been cut out from a normal- 

 sized wing. This means that the wing edge and wing veins are really 

 absent where a piece of wing spread is missing. The extreme vestigial 

 (and a more extreme No wing), therefore, has in its wing stump only 

 the more or less normal venation of the wing bases of a normal wing 

 and not a reduced venation of a whole wing apart from some minor 

 features of regulation. I found that the lower members of the series 

 had a normal wing at the time of pupation, and only later, when the 

 wing anlage contracts (in all cases), the scalloping appears. I inter- 

 preted this as a lysis of already formed wing material in analogy with 

 well-known cases of such lysis in Lepidoptera (Hterature in Gold- 

 schmidt, 1940). In the higher alleles (with more scalloping) the wing 

 is already incomplete at pupation time, a fact which I interpreted as 

 owing to earlier lysis. In vg itself even the early imaginal disc is 

 smaller (Chen), which meant to me that lysis had started very early. 

 Waddington (1940a) gave a different description of the facts. He 

 thinks that the contraction of the pupal wing is also responsible for the 

 scalloping in the lower grades. To explain the higher ones, he assumes 

 a strange migration of tissue around the edge of the wing, a process 

 which he himself characterizes as rather unbelievable. Actually, this 

 explanation fails to explain the determination of the venation, which 

 is the normal venation for what is left of the wing. Thus the actual 

 happenings must be either as I assumed it or of a comparable type, and 

 therefore the embryonic action may be described in terms of produc- 

 tion of a lytic substance or by the absence of a necessary growth sub- 

 stance — an assumption which easily could be modified, while still 

 working in the same direction, if further work showed that another 

 developmental feature is responsible for the effect. 



The elementary genetic facts can now be described, following 

 Mohr ( 1933 ) , by assuming different potencies for the different alleles, 

 meaning the ability to produce different amounts of growth sub- 

 stance, to use the simplest model. Let us assume that the threshold 

 value for wild-type wing is a quantity of 40, and therefore the differ- 



