Ill INDUCTION GENERAL REMARKS 475 



sorption of active substances, i.e. molecules with definite appropriate configurations. 

 Some cases of induction at a distance or through a fluid medium (p. 409) have 

 been observed. With dead inductors, the topographical relations may often be 

 best interpreted by imagining a diffusion gradient from the implant (Fig. 89, 

 p. 438). With living inductors, a similar relation can be sometimes observed in 

 vivo, as Pasteels did when studying the influence of the primary embryo on a 

 secondary one obtained by centrifugation of the blastula (p. 414). The crucial 

 test of transmission at a distance was effectively satisfied, at least for a definite 

 secondary induction,' by the use of adequate filters, and the objection could be 

 eliminated (p. 465) that a contact by thin cytoplasmic filaments penetrating the 

 pores was necessary. 



The appreciable benefit from this discussion is a better understanding of the 

 different possible mechanisms for the transmission of an inductive agent (Fig. 96, 

 p. 464). My impression is that a transfer of organic molecules may be considered 

 certain, but that their transmission through a fluid medium is only experimental, 

 the normal process taking advantage of close contact and probably of the cellular 

 intercellular matrix. 



The biochemical nature of the transmitted agents is, at least for many workers, 

 the goal of these investigations. Fractionation of xeno-inductors has been much 

 used in this purpose but, as we have seen in the discussion of these investigations, 

 it is doubtful that they could reveal the nature of the agents at play in normal 

 induction. For this, the normal process should be studied in itself, and this has 

 been done at least in some instances. 



Cytochemical methods have been accurately applied to primary and secondary 

 inductions, and they have uniformly drawn the attention to two components, the 

 ribonucleoproteins and the alkaline phosphatase. The active synthesis of this 

 enzyme can be shown, on adequate material, as occurring in the reactor, and is 

 thus more a consequence than a cause of induction. By contrast, changes in the 

 RNA reaction are observed as well in the inductor as in the reactor, as first 

 recognized by J. Brachet. Of course, in xeno-inductions, it has been repeatedly 

 demonstrated that RNA is not the active fraction, but Brachet was right when he 

 remarked (Brachet, Kuusi and Gothic, 1952, p. 439) that these criticisms do not 

 invalidate the relations he had formerly stressed in normal development. More- 

 over, the variations in RNA content may be thought to concern, in the living cells, 

 ribonucleoproteins. 



The RNA of reactor and inductor at various stages has been measured, as 

 related above (Fig. 36, p. 362) by Rounds and Flickinger. Their results agree with 

 the preceding idea, except that the increase in the reactor does not really coin- 

 cide with the decrease in the inductor. It seems thus that induction is not only a 

 transfer of definite agents^, but also the awakening of various syntheses in the reactor 

 cells, according to my concept of metabolic contagion (1940). The introduction of 

 foreign macromolecules in the induced cells would not be simply an enrichment 

 by addition, but the release of new syntheses, an idea also advocated by Nieuwkoop, 



' For primary induction, a new tentative by Brahma (1958) of interposing a filter failed. 

 ^ Possibly in very small amounts (see p. 363). 



Literature p. 4fS3 



