DIFFERENTIAL GROWTH I55 



US say, compounds e in the cell exposed to the E surface, and compounds 

 <t> in that at the F surface, where e and </> may be entirely unrelated. 

 According to the preceding section, such adsorbed and oriented border 

 species can exert key effects in turning the chemical reactions, segrega- 

 tions, and syntheses among the rest of the population into a definite 

 course. Consequently the chemical fates of the two cells, coated by 

 qualitatively different key species, will likewise become qualitatively 

 different. Thus a difference which started out as merely one of distribu- 

 tion gradually develops into one of composition and character. This 

 would be the fundamental step in the dichotomy of cell fates. This con- 

 cept remains, for the present, hypothetical. But being more concrete and 

 specific than the conventional verbal concepts, it has the advantage over 

 the latter of being amenable to experimental verification. Tentatively, it 

 fits the known facts of development very satisfactorily, as will be shown 

 in the following. 



On an earlier occasion (47) I outlined three basic principles of 

 cytodifferentiation, which I called (i) discreteness, (2) exclusivity, and 

 (3) genetic limitation. Any valid concept of differentiation must be 

 consistent with these principles. 



The principle of discreteness refers to the fact that "differentiation 

 produces a definite number of discrete, distinct, discontinuous and more 

 or less sharply delimited cell types which are not connected by inter- 

 gradations." The principle of exclusivity states that once one type of 

 differentiation has taken hold of a cell, no fraction of such a cell can 

 ever become engaged concurrently in any other type of differentiation ; 

 the cell in differentiation acts as an entity in all-or-none fashion. For 

 instance, there is a sharp dividing line at the rim of the optic cup sepa- 

 rating optic retina from tapetum ; one cell is a typical retinal cell and the 

 next cell is just as typical a pigmented tapetum cell. Although we know 

 from experimental evidence that originally both have the endowment to 

 transform into either type, they never form a half-way blend. Examples 

 of this kind could be multiplied at will. It is evident that these principles, 

 derived from empirical facts, can be logically deduced from the postu- 

 lated discreteness of the molecular key species «, P, y, 8, e, and their 

 mutual exclusiveness in the competition for surface positions. Thus, a 

 given cell must follow either the a-determined or the /3-determined or 

 the y-determined, etc., course, and follow it wholly and with no compro- 

 mises. 



The third principle, genetic limitation, stating that each cytodif- 

 ferentiation occurs according to the special methods characteristic of 



