468 GERMINAL ORGANIZATION INDUCTION PHENOMENA 4 



(a) About definition 



Typical induction, as embryologists understand it, exists whenever the fate of a 

 cell-layer or group is influenced by another adjacent cell population. The nature 

 of the influence, being still disputable, cannot be taken as the criterion for a more 

 strict definition. There are doubtful cases^ in which the older expression of correla- 

 tion seems sufficient, as when perichordal mesenchyme condenses into the chordal 

 sheath. In apparently similar cases, however, such as around sensory organs, 

 where the condensing mesenchyme is the precursor of the cartilage capsule, this 

 differentiation is more than simply a correlation since an induction is responsible 

 for it. The change in the fate of the reactor also implies that it is still malleable, 

 or, as Waddington says, competent. After so-called "determination" has taken place, 

 some possibility of modulation in the mode of cytodifTerentiation may still remain 

 (Weiss, 1952), as when, in an eye grafted with lung tissue, cornea cells proliferate 

 into epiderm nodules which become keratinized (May et al., 1957). Such an event 

 is necessarily exceptional: inductive changes of structure are not observed in non- 

 embryonic tissues. The existence of the xeno-inductors could at first sight be 

 interpreted as if the inducing principle(s) subsist in the various organs through- 

 out life, but this idea, as we did see above, does not resist to critical examination. 

 The phenomenon of xeno-induction can be interpreted otherwise (p. 447). 



Thus, manifestations of induction are numerous and manifold, with many 

 normal, experimental, teratological and even pathological aspects, but the process 

 remains fundamentally a developmental mechanism. Beyond a certain stage, the 

 induced tissue acquires full autonomy, evident by the fact that in pure culture it 

 will generally "breed true". Nevertheless, the possibility is not excluded that some 

 structures could depend for their maintenance upon the presence of their inductor 

 or even of an organ which they have induced, as for example, the large power of 

 regeneration inside the embryonic nervous axis or the lens-retina relation. As 

 expressed by Bautzmann (1951), the inductor and the reactor form a functional 

 couple. 



(b) On the evolution of inductive processes 



From the evolutionary point of view, inductive mechanisms appear to be a 

 secondary acquisition. Chordates most probably descended from certain Metazoa 

 in which these devices may have been present (or potentially prepared), but not 

 morphologically active. The protochordate ancestors did use them to some 

 extent (p. 355) but their intensive utilization can be thought to have appeared 

 progressively in vertebrates. This progression is patent in the fact that the repro- 

 ductive cycle itself has rapidly become dependent upon induced structures, and 

 only a few lower groups still show direct shedding of eggs by a genital pore. Re- 

 peated improvement of inductive mechanisms, with their extension to organs of 

 circulation (heart), excretion (meso- and metanephros), respiration (branchial 



^ Such are also several interactions between neural plate, notochord, somites. It has been 

 considered that the neural plate or the spinal cord plays a role in the segmentation of the 

 mesoblast (Ragozina, 1946), that the notochord has a similar influence (Takaya, 1956a), 

 and that the same organ limits the dispersion of the neural crest cells (Tada, cited from 

 Fujii, 1944). 



