Ill INDUCTION GENERAL REMARKS 47 1 



into several kinds of mixed cells, calls for caution, as does the inlerdependance of 

 rudiments inside the middle layer. It might well be that, in the beginning, differ- 

 entiation always depends on exchanges with the intercellular medium, itself 

 conditioned by neighboring cells, while later a metabolic specialization calls forth 

 the so-called determination. 



Whatever it may be, typical induction leads to differentiation not only in the 

 neuro-epithelium, or in its secondary derivatives, retina and tapetum, but 

 also in the lens, the olfactory and auditive epithelia, lateral line organs, limbs, 

 mesonephros, metanephros, and all kinds of glands. Thus normal induction 

 provides the conditions not only for the shaping of the rudiments but also for their 

 functional difTerentiation, which is the real aim of the process. 



Clearly conscious of these effects of induction we may now attack the core of the 

 problem as it stands today : the individuation of the neural axis into a prechordal 

 and chordal part. By the preceding remarks, and also by those on p. 408, we are 

 prepared to admit that the differences are not abrupt, but gradual, mostly related 

 to quantitative variations in the concentration of cells, the intensity of proliferation 

 the extent of mass movements, the progression of cyto-differentiation, the dis- 

 persal into ectomesenchyme, and the amount of melanin produced. Even such 

 remarkable layers as the retina and tapetum are not so different from other neuro- 

 epithelial derivatives: at the onset, the retina makes an impression only by the 

 number of neuroblast layers and the orderliness of their concentric differentiation, 

 while tapetum behaves nearly as neural crest cells. On the other hand, inspection 

 of neural development and the course of many experiments testify that the character- 

 istics of the various regions are not acquired at once, as if released by some trigger 

 mechanism, but evolve progressively. 



Although practically all authors concerned with induction have expressed more 

 or less personal views on this problem, nowadays three or four main positions may 

 be distinguished and considered without reference to their preliminary or side 

 aspects. 



The one proposed by Yamada (1950b) and repeatedly advocated by its author and his 

 coworkers admits the distinction between a dorso-ventral morphogenetic potential (jP. ^y) 

 and a caudo-cephalic one {P. cc), decreasing as suggested by these expressions. It was 

 an original and important step to consider the second gradient as increasing from head 

 to tail, in opposition with the current assumption. The same idea must also be accredited 

 to F. E. Lehmann (1950). To represent the factors which confer to each gradient the values 

 attained at each level during the successive phases of early development, Yamada invokes 

 mediators (a dorso-ventral and a caudo-cephalic one, M. dv and M. cc) which reflect, trans- 

 late or sum up the normal or the experimental conditions ruling individuation. The 

 representation is certainly supple and suggestive but implies a fundamental duality in the 

 morphogenetic tendencies of the normal egg. The factors represented by P. dv and M. dv 

 are supposed to be inherent to the presumptive ectoblast, the P. cc and M. cc agents to the 

 chordomesoblast. 



The interpretation proposed by Nieuwkoop et al. (1952) stresses the auto-organizing 

 tendency of the activated ectoblast. In his opinion, this process is a release mechanism, 

 which is necessarily the first step of all neuralization. Left to itself, the neuroepithelium 

 essentially proceeds to build up a prosencephalon. The formation of other parts of the neural 

 axis depends on transformation agents which are normally produced by the chordomeso- 

 blast. Thus, normal neurogenic induction is due to the succession of activation and transfor- 



Literalure p. 483 



