DIFFERENTIAL GROWTH I 59 



and orientation. Corresponding molecules of complementary configura- 

 tion would be present in the ectoderm cells ; but during the earlier labile 

 phase of these cells, these would still lie scattered in the interior, mixed 

 with an assortment of other species. As the organized substratum then 

 doubles under the ectoderm, its surface molecules would attract their 

 molecular counterparts in the ectoderm cells into the contact surface. 

 From their oriented surface positions, these molecular layers in the 

 basal face of the ectoderm cells would then control the further chemical 

 developments in their cell bodies, as outlined above. It can readily be 

 seen that the type of response to be obtained from such an inductive 

 action depends both on the character of the substratum and the types of 

 molecular species present in the reacting cells at the time of exposure. 

 The progressive transformation of the molecular equipment of the cell 

 explains why there is often a limited period of "competence" (43) dur- 

 ing which cells can appropriately conform to a given inductive stimulus. 



In contrast to previous hypotheses, which for the most part have 

 ascribed the inductive effect to diffusible "evocator" substances, the 

 theory here advanced presumes a definite spatial organization of the 

 molecular population as essential for inductive success. Induction of 

 this type could be transmitted only by direct contact. This is in agree- 

 ment with experience. Lens induction requires intimate adhesion between 

 retina and epidermis. Neural plate arises in direct contact with the 

 chorda plate. Many more examples could be cited that would seem 

 to discount the diffusible nature of the inductive agent. One might 

 object that neural inductions have been obtained in explants floating in 

 body fluids, but it is conceivable that in these cases the active role is 

 played by a protein film adsorbed to existing surfaces to which the 

 explant has temporarily adhered, or a similar film settling along the 

 outer surface of the cells. Since there is a direct relation between the 

 presence of disintegrating cells and the occurrence of neural inductions 

 (18), it is plausible to assume that some neural key species, set free 

 from the destroyed cells, collect and become oriented along the outside 

 of the surviving cells and thereby become an inductive film in the sense 

 discussed under homoeoinduction. 



These comments are conjectural. There are not enough facts known 

 on which to decide the issue of contact versus distance action conclu- 

 sively. It stands to reason that a precise and objective description of the 

 cytological changes attending induction, together with experimental tests 

 of their relevance for the process, could provide some of the wanting 

 clues. A suggestive instance is the following. In observing tissues in 



