Genie Control of Development 405 



(Piepho, 1942), pieces of skin of just hatched lepidopteran larvae 

 can pupate in the absence of the juvenile hormone, it seems that here 

 competence of the substrate is not reached by a progressing condition 

 during larval life. But if in Drosophila (Bodenstein, 1943) different 

 discs become competent for reaction to hormones in definite order, 

 this clearly shows a phenomenon of independently determined com- 

 petence. The latter case is of special importance for our analysis of 

 the role of the hormones within the system of genie actions. A very 

 good case has been made by Bodenstein ( 1953i> ) , who showed that in 

 Periplaneta the formative material becomes increasingly susceptible 

 with age to the differentiation promoting prothoracic gland hormone 

 up to the adult condition. This is shown when the genital apparatus 

 of an adult female is transplanted into a younger host and actually 

 molts, while remaining adult; whereas a genital anlage of a not yet 

 adult female, in the same situation, molts almost like a larval 

 (nymphal) structure. A number of comparable cases are known, and, 

 under special experimental conditions, it is even possible that the 

 opposite happens : under corpus allatum influence, adult skin may molt 

 into nymphal skin ( Wigglesworth ) . 



One more point in the interplay of these hormones is relevant to 

 our discussion. According to Wigglesworth, the prothoracic gland 

 hormone (the differentiation-inducing one) is released before the 

 juvenile hormone in the initiation of molting; consequently the devel- 

 oping tissues are subjected for a short time to the differentiation 

 hormone alone, which provides the stimulus for the progress of dif- 

 ferentiation from stage to stage, while the succeeding juvenile hor- 

 mone inhibits the process up to the time of metamorphosis, when it is 

 obviously absent or in too low concentration. Thus a remarkable 

 interplay of these hormones pulling in different directions is estab- 

 lished. There is an interesting corollary to this. We mentioned the 

 extra molt of adult structures (genitalia) under the influence of the 

 molting hormone, but the resulting structure was the same as before; 

 that is, no further differentiation was possible. But Bodenstein 

 (1953fc) also found organs (the sternum of Periplaneta) which molted 

 into a superadult condition, going in the same direction beyond the 

 adult stage. Clearly this is an example of different substrates reacting 

 to the same stimulus. 



We come now to what is known on the genetic side. Molting in 

 Lepidoptera is also controlled genetically, since there are races that 

 differ according to the number of larval molts. In the silkworm, races 

 with three to five molts are known (see Tanaka's review, 1953). In 



