Genie Control of Development 315 



latter in most organ determinations. In the former, the different fate 

 of two division products of a cell could be described in Weiss' terms 

 of separation of molecular populations. In the latter, a supracellular 

 area would receive different molecular populations from the adjacent 

 one. I cannot see how the description in terms of molecular populations 

 would in either case help us in understanding the genie control of this 

 process and its action according to the pattern in space and time (i.e., 

 the old problem of genie activation). It merely replaces the term 

 "chemodifferentiation" by the expression "diversification of molecular 

 populations." Since differentiation (on all levels) implies in the end an 

 irreversible change (or one reversible only in very unusual circum- 

 stances), this must be on the molecular level. But how the genie 

 material affects this differentiation remains the unsolved problem. We 

 still wish to know whether the change in what may be called com- 

 petence of a cell group is an automatic sorting out of specific con- 

 stituents of the cellular substratum by a process of "stratification" 

 enforced by the physical and chemical conditions of the cellular 

 system as a result of its past history, that is, initiated at the beginning 

 of development (which would include also possible hereditary cyto- 

 plasmic conditions or components); or whether this segregation of 

 materials or physical conditions or both requires its own genie action 

 at the moment it occurs. In other words, does the change in com- 

 petence between two cells or cell groups, which permits the begin- 

 ning of specific action of the genie material by providing for it the 

 proper substrate, follow automatically from the conditions reached at 

 the moment as a consequence of former developmental steps, which 

 would be consecutive diversification? Or is it necessary that a genically 

 controlled product starts the "stratification" by producing some initial 

 reaction? As I see it, the idea of molecular ecology does not help us 

 to understand this basic situation, but only circumscribes its existence 

 in different terms. (Lwoff, 1950b, seems to have no such scruples.) 

 We have already discussed the ideas of Brachet and others which 

 solve this problem by assuming a mixture of seff-reproducing specific 

 cytoplasmic elements, sometimes, unfortunately, called plasmagenes, 

 which are, in a way, both autonomous and dependent upon the genie 

 material. The change of competence (in the wider sense of the term 

 as used here) of cells or cell groups would have to mean sorting out 

 of these elements, which leaves us again where we were, in need of 

 some kind of stratification. As I stated previously, the underlying facts 

 ( the work on cytoplasmic RNA ) point more in the direction of growth 

 as the proper function of these cytoplasmic particles. Weiss also 



