314 Action of the Genetic Material 



shall review critically some more work of this nature before trying to 

 formulate a general idea commensurate with today's knowledge, and 

 shall begin with tlie analysis made by embryologists. The aspect 

 emphasized in Brachet's work will on this occasion reappear. 



b. Specific ideas derived from embryology 



An elaborate attempt to understand genically controlled dif- 

 ferentiation has been made by Paul Weiss (see 1950). He draws on 

 all aspects of experimental embryology, though the genetic side 

 remains rather in the background. The basis of his ideas is the concept 

 of "molecular ecology" within the individual cells. Each cell and cell 

 part consists of an array of molecular species, whose densities, 

 distribution, arrangement, and groupings are determined by their own 

 properties as well as those of their surroundings. Chemical segregation 

 and localization within the cell result from free molecular interplay, 

 as only groups of elements compatible with one another and their 

 environment can form durable unions. Interfaces play a considerable 

 role as segregative factors of molecular mixtures. The fixation of a 

 particular molecular species in a surface is partly due to unspecific 

 factors (like adsorption), but also to selective chemical affinities. 

 Thus morphogenesis, which is the development of structure in space, 

 means the proper segregation of the members of the molecular popu- 

 lation within the cell. I think it is evident that this concept does for 

 intracellular differentiation, in terms of molecules, what our concept 

 of "stratification according to the conditions of the system" (i.e., 

 physical and chemical nature of the whole) does for the regional 

 determinations in the developing egg. It is considered safer, however, 

 not to allow oneself to believe that a description in the language of 

 molecules has much meaning without biochemical specification of 

 these molecules and their interrelations. From the genetical point 

 of view, the intracellular differentiation is, however, the minor 

 problem. 



We have already seen that specific differentiation like that of the 

 wing-scale cell may be combined with a cessation of the specific genie 

 functions. However, the determination of the embryonic primordia at 

 the proper time and place is the real function of the genie material. 

 Theoretically, this primary differentiation may occur in a differential 

 division of one primary cell in regard to its cytoplasm, but it may also 

 be the result of separation of cytoplasmic qualities over an area of 

 many cells, independent of cell limits. Probably both types occur, the 

 former in the meristemic type of differentiation (see III 5 A a); the 



