l6o PAUL WEISS 



inductive interaction I have often been struck by the remarkable corre- 

 spondence of their cell and nuclear orientation just during the crucial 

 phase. This phenomenon is very marked, for instance, in the formation 

 of the lens. When the eye cup makes contact with the overlying epi- 

 dermis, the irregular cells of the latter, within an area exactly coexten- 

 sive with the area of contact, turn into a prevalent radial orientation, 

 each elongate nucleus lining up in the direction of the axis of the 

 opposite retinal cell. Precisely these oriented cells will later constitute 

 the lens placode. The orientation effect is transitory, being obliterated 

 by the subsequent transformations of the placode. While experimental 

 tests remain to be applied, the cell-sharp coincidence between area of 

 contact, orientation, and lens determination makes it almost certain that 

 the observed cell changes are directly related to the primary inductive 

 effect. Evidently, what happens is that the content of the epidermal 

 cells undergoes a thorough reshuffling, with the underlying retinal cells 

 acting as attractive foci. In short, the elongation and orientation of cell 

 structure would be visible expressions of the oriented shifts of the 

 molecular population, which would have to be expected from our theory. 

 A systematic investigation of this orientation phenomenon is under way. 



The foregoing comments on induction were presented mainly as 

 illustrations of how problems of development gain in sharpness when 

 resolved into terms of molecular behavior. The special solutions sug- 

 gested should be considered merely as a first crude and wholly tentative 

 effort to replace the vague, noncommittal, and unrealistic notion of 

 "inductor substances" by a specific, concrete, and testable concept. The 

 limitations of this concept are self-evident. Therefore our insistence lies 

 more on the merits of the particular manner of viewing the phenomena 

 than on the tentative views presented. 



As for the limitations, one must not lose sight of the fact that the 

 concept pertains only to the instrumentalities of biological processes 

 and not to the overall order which these processes follow and which we 

 usually refer to as organization. It can explain the manifest differentia- 

 tion of a given molecular population, provided only that there is a regu- 

 lar frame of organized surface conditions to start with. The organiza- 

 tion of the cell content is thus referred back to a prior topographical 

 organization of the various surfaces of cell, nucleus, chromosomes, etc., 

 and the principle, omnis organisatio ex organisatione (48), still holds. 

 The "field" character (47) of the organizing conditions in develop- 

 ment, which cannot be reduced to molecular terms, has not been touched 

 at all in our discussion. 



