Genie Control of Development 417 



diffusible substances, and so on. To these must be added the setting 

 in motion of the regulatory processes which seem to be a primary 

 property of all cells in contact with each other. These processes, if set 

 in motion in a definite way by genically controlled embryonic situ- 

 ations, might provide another short cut for genie actions, meaning 

 unavoidable (pleiotropic) consequences of these actions which extend 

 and therefore simplify the picture of genie interplay. 



Some data relating to the present discussion have been presented 

 by Waddington (1953fo) in his study of interaction of different 

 mutants. As far as the facts go, they are in general agreement with 

 those found in the studies of homoeotic mutants. A special study of 

 genetics and embryology of some imaginal discs of Drosophila by 

 Hadorn (see 1948; and Hadorn and Gloor, 1946), comes still nearer 

 to our discussion, though the details are infinitely more simple. The 

 experiments dealt with the genital disc, which in the female produces 

 a number of internal and external sex organs, visible only at meta- 

 morphosis: uterus, oviduct, the single receptaculum seminis, the two 

 spermathecae and parovaria, and further, on the surface, the two 

 vaginal plates and the anal plates. The fields for all these differenti- 

 ations are finally determined as early as the third larval stage. This 

 was shown by cutting the disc in pieces and reimplanting them: each 

 gave only what it would have given within the whole. At one point, 

 however, a regulation was found possible: when a small exterior 

 piece was cut off, it could produce more vaginal plates and spines 

 than it should. The cut produced a "raw edge" which induced some 

 regulation. The male genital disc, however, is capable of much more 

 regulation. 



There is a mutant called "spermatheca" which affects only the 

 organ of its name: either the ducts of the two spermathecae unite; or 

 the paired organs unite in one; or three organs are formed. Each type 

 can be obtained alone by temperature treatment at definite times. This 

 shows that number and form of spermathecae in the mutant are 

 determined finally only in the second half of the third larval instar, 

 though in the wild form the experiments reported above show that the 

 disc was already finally determined at this time. Hence it must be 

 assumed that the mutant spt prolongs the labile state of determination; 

 that is, it acts directly upon a time or threshold element in the system 

 of processes called embryonic determination. Thus, in principle, the 

 system of genie actions is comparable to that described for the pod 

 and tet mutants, though on a simpler scale. 



