Genie Control of Development 347 



start of cell divisions with simultaneous end, the first, of course, pro- 

 portional to dosage, compared with our deduction of a simultaneous 

 start, but inhibition sooner or later according to dosage. According to 

 Steinberg (1941), the decisive first divisions in development are not 

 known; they occur before the temperature-efiFective period. Facet 

 formation (which takes place much later) is delayed in B flies as 

 compared with + flies; later on, the growth curves are identical. 

 Obviously, the Bar effect controls only the number of primordial cells 

 (Steinberg's cell capital). Thus we do not know whether the Bar 

 condition affects the growth substances (hormones) controlling the 

 primordial cell divisions so that divisions begin later in Bar and later 

 still in higher combinations. Since the effect is additive, proportional 

 to dosage, this might mean only an inhibition, for example, of the 

 speed of production of these "hormones," that is, interference with a 

 kinetic feature of genie action. The other possibility is that the 

 production of these hormones always starts normally but is exhausted 

 at different times according to dosage. This could again be inter- 

 ference with the kinetics of production; but it might also be conceived 

 as a direct action upon the control of a threshold condition necessary 

 for continued cell divisions. One of the possibilities of visualizing this 

 would be the assumption that the threshold is set by determination of 

 some of the cells as head epidermis, a process of a completely dif- 

 ferent kind, interfering with facet formation. In the end, however, 

 this secondary inhibiting action via redetermination would also work 

 in some way via kinetics of a process, as it has to spread with dosage. 

 This would be a third possibility, in general terms, inhibition by an 

 outside process or product proportional to B dosage. There are some 

 facts bearing upon the choice among these possibilities. Thus tem- 

 perature effects upon facet number can still take place after estab- 

 lishment of the primary differences. This would mean, in a general 

 way, that the processes of inhibition of cell division are of a kinetic 

 type, having a temperature coefficient. Another fact (Luce, 1941; 

 Williams, 1945) is that an experimental increase of developmental 

 time produces more facets. This fact again can be cited in favor of all 

 the possibilities mentioned. Thus we see that it is very difficult to 

 derive one exclusive interpretation of these dosage effects, but, in a 

 general way, they point to action of the genie material upon some 

 phase of the kinetic processes in development rather than to a quah- 

 tative chemical action. 



However, there are some other facts which lead a step farther. 

 It seems that the number of divisions of the primordial cells is only 



