348 Action of the Genetic Material 



one factor in the final determination of the size of the Bar effect. It is 

 known that at a much later time of development, when the aggregation 

 of the primordial cells into pro-facets takes place, the number of facets 

 resulting can be greatly influenced (during the critical period) by 

 temperature action, X rays, and other agents, mostly in the direction 

 of fewer facets. Since the primordial division period is already over, 

 this cannot be an effect upon the kinetics of cell division. The decisive 

 experiment is due to Chevais (1943). He found in Calliphora larvae 

 an extractable substance which has to do with ommatidia differ- 

 entiation. If enough of it is injected into Bar-eye larvae before dif- 

 ferentiation of the ommatidia, a completely normal eye can be pro- 

 duced. This shows that the primary determination of the number 

 of cells as facet primordia is only one aspect, though certainly the 

 major one. The other side is that the surrounding cells of the imaginal 

 disc which are destined to be determined as head epithelium still 

 retain their potency to develop into facets. Since it is an extractable 

 substance which can enforce this, it might be concluded that the 

 primordial cells or the already grouped pro-facet cells produce a 

 substance which prevents other cells of the head epithelium from 

 developing into facets. This would be another Bar effect, but it can- 

 not be the one which shows the dosage difference. 



This interpretation of the facts is, however, not the one preferred 

 by Luce (see Luce et ah, 1951). He shifts the emphasis completely to 

 the processes in the critical period, and thinks that the primary 

 difference in cell numbers is of no importance and that everything 

 depends upon the number of disc cells which in the much later 

 sensitive period are determined for epidermis or facets. Therefore, the 

 cells of the Bar mutants are deficient in a substance which is a 

 necessary precursor for the initiation of differentiation into ommatidia. 

 The dosage effect, then, would be one upon the amount of deficiency 

 of this substance, according to Luce. I cannot see any advantage in 

 this interpretation and prefer to consider the processes in the sensitive 

 period as a secondary effect. It seems to me that the majority of facts 

 fit this explanation. However, this discussion shows that the details of 

 the processes between dosage of causative agent and quantity of effect 

 are anything but simple, though, in a general way, they can be 

 described as effects upon the kinetics of chains of reactions. 



In recent years. Stern and collaborators (1943-1948) have tried 

 to gain more detailed insight into genie action from dosage phenom- 

 ena. The large body of facts is anything but simple, and at some 

 points great difficulties arise for a unified explanation, a difficulty 



