Amphibians 



255 



elements have occurred in most instances in 

 combination with parts of the central nervous 

 system. In fact, there is hardly any case 

 known in which induced neural tissue has 

 not been accompanied by neural crest deriva- 

 tives. 



Dalcq ('41a, '46) and Raven and Kloos 

 ('45) have interpreted these results in terms 

 of a concentration gradient of a hypothetical 

 inductive substance, "organisine," present in 

 the archenteron roof (see p. 278). The 

 median strip of the archenteron roof, sup- 

 posedly rich in organisine, would induce 

 neural structures while the more lateral parts 

 which elaborate it in smaller quantities 

 would induce neural crest. 



There is little evidence to support this 

 gradient concept. Whereas the experiments 

 of Bautzmann ('28, '29) seemed to indicate 

 that the prospective notochord has a stronger 

 neuralizing effect than the prospective so- 

 mites, the experiments mentioned above and 

 those of Bytinski-Salz ('31) did not confirm 

 this contention. They have shown that the 

 frequency and amount of neural tissue in- 

 duced by prospective somites is not inferior 

 to that obtained by prospective notochord. 

 The embryos, in which the formation of 

 large sections of the notochord was sup- 

 pressed by chemical treatment without any 

 sign of reduction of the neural tube (F. E. 

 Lehmann, '35, '37, '38; Lehmann and Ris, 

 '38), gave further evidence that the prospec- 

 tive somites and the notochord have equal 

 potencies as inductors. The stvidies of Raven 

 and Kloos ('45) indicate that neural crest 

 derivatives are induced equally well by 

 median and lateral pieces of the archenteron 

 roof, whereas neural tissue was induced more 

 frequently by the former than the latter. 

 However, the small number of the experi- 

 ments does not permit a statistical evalua- 

 tion. Furthermore, the absence of neural 

 inductions by lateral mesoderm can be cor- 

 related with a very poor differentiation of 

 the transplants themselves. The available 

 evidence thus indicates merely that some 

 neural crest derivatives can be induced in 

 the absence of a neural plate and that, if this 

 happens, paraxial mesoderm was responsible 

 for it. As a matter of principle, the question 

 of whether the emergence of different strvic- 

 tures is due to qualitative or quantitative 

 differences in inductive substances can only 

 be answered by experiments in which such 

 substances are isolated and applied in graded 

 dosages. Experiments with normal inductive 

 tissues can never answer this question satis- 

 factorily. 



At the stage of rising neural folds, a 

 regional pattern of limited differentiation 

 capacities is already laid down in the neural 

 crest: The precursors of melanophores origi- 

 nate mainly from the trunk part of the crest 

 (Niu, '47, for Triturus) and those of visceral 

 cartilage exclusively from the head crest 

 (Harrison, '25; Raven, '31, '33; Horstadius 

 and Sellman, '45). Little is known concern- 

 ing the determination of these patterns. How- 

 ever, final differentiation of both types of 

 precursor cells depends on additional factors 

 residing outside the archenteron roof and the 

 neural crest itself. The chromatophore pat- 

 tern will be discussed elsewhere. The forma- 

 tion of visceral cartilages requires an activa- 

 tion of their precursor cells by pharyngeal 

 entoderm or, under experimental conditions, 

 by notochord or intestinal wall of the trunk 

 (Horstadius and Sellman, '45; and others). 

 In a similar way, final differentiation and 

 functioning of the other derivatives of the 

 neural crest seem to depend on additional 

 stimuli which are ordinarily furnished at 

 the sites where the migratory crest cells 

 settle down. Thvis, the formation of myelin 

 depends on the establishment of contact be- 

 tween Schwann's sheath cells and nerve 

 fibers, and the crest cells migrating into 

 the mouth region produce dentine only 

 when in contact with oral ecto- or entoderm. 

 One may hesitate to give these additional 

 environmental factors the same rank as the 

 initial ones, since it is possible that at least 

 some of them are merely necessary to realize 

 the potencies evoked already by the initial 

 inductive factors. 



STRUCTURES OUTSIDE THE 

 MEDULLARY PLATE 



The situation is similar with respect to 

 the placode derivatives: nose, lens and oto- 

 cyst, and some other structures. In these 

 instances, several inductive tissues are in- 

 volved which operate either simultaneously 

 or in succession, supplementing and rein- 

 forcing each other. These inductions thus 

 serve as new illustrations of an important 

 general principle; namely, of the existence 

 of synergistically active ^'inductor systems" 

 (Holtfreter, '35a,b). We have already en- 

 countered this principle in the case of the in- 

 duction of spinal cord by the chorda-somite 

 system and in the case of the brain-eye 

 induction by a multiplicity of primordia of 

 the anterior archenteron roof. In otocyst in- 

 duction, a conditioning of the ectoderm by 

 lateral head mesoderm is followed by an 



