GENERAL CONCLUSIONS 243 



the differentiation of a neural tube. The chemical nature 

 of these differentiating substances or ''evocators" (Wadding- 

 ton, 1934) does not appear to be highly specific. Fischer et al. 

 (1935) have found that similar effects can be produced by 

 highly purified oleic acid, hnolenic acid, and 12-octadecenic 

 acid-1, as well as by muscle adenyUc acid. Saturated acids, 

 including stearic acid, were not active. From these results 

 they conclude that the induction can be regarded as a 

 stimulation by acid ("Saure-Reiz"). Such a stimulation 

 might be interpreted in the same way as the effect of acid 

 on growth of plants, namely that the acids set free a certain 

 amount of active substance, but have no activity themselves 

 (cf. VIII F). The failure of the saturated acids to act may 

 be due to their lower ability to enter the cell. The results of 

 Waddington, Needham, et al. (1935) cannot be interpreted 

 in this way, and the active sterols and hydrocarbons studied 

 by them apparently possess evocator acti\aty of their own. 



It must be emphasized that such evocations are not to 

 be confused with the action of the living organizer, which not 

 only differentiates the neural tube but also controls all 

 other differentiations, i.e. it imposes a pattern on the em- 

 bryo. This pattern-formation seems to call for the assump- 

 tion of some kind of "field," of the same type as the polarity 

 of auxin transport in the plant. 



In organization in the plant, such as root formation, the 

 ''field" is the polar transport of the active substances; the 

 evocator itself can be compared with auxin. Although auxin 

 is formed or present in many different tissues, organization 

 takes place only in that special location which is determined 

 by the polar accumulation of the auxin. Correspondingly, 

 in animal embryos, the evocator may be present in other 

 parts of the embryo, as Holtfreter (1935) showed; the lack 

 of a "field," however, is probably the reason why it does 

 not act there. There is thus a close parallelism between 

 auxin and the evocator; both are present in many parts 

 of the organism, but their effects are limited by an existing 

 "field," i.e. by their polarized accumulation. As to the 



