Amphibians 



279 



dalizing and others a somite-producing ef- 

 fect. This seems to suggest that qualitative 

 rather than quantitative differences in the 

 inductive agents cause these divergent dif- 

 ferentiations. 



It should be pointed out that, so far, no 

 purified tissue extracts or chemicals have 

 been found which transform ectoderm into 

 mesodermal tissues, and the claim that the 

 mesodermizing agent may be represented by 

 proteins and the neuralizing agent by nu- 

 cleic acids (Brachet, '45, '50; Kuusi, '51) 

 is based on inconclusive data. 



Since it has not been possible to convert 

 neuralizing tissues or chemicals into in- 

 ductors for non-neural structures, e.g., for 

 those of lens, ear or teeth, it is likely that 

 the latter owe their emergence to the action 

 of still other agents. No doubt, temporal 

 and regional changes of competence are 

 involved in the distribution of these sec- 

 ondary inductions, but it is questionable 

 whether the principle of competence is suf- 

 ficient to account for this regional diversity. 

 Especially in cases of a two-step inductive 

 mechanism, as is exemplified in the deter- 

 mination of teeth, branchial cartilage and 

 pituitary, where the histogenetic effect of 

 the first stimulus differs strikingly from that 

 of the subsequent stimulus, it seems plausi- 

 ble to assume that successively different in- 

 ductive agents are engaged. 



THE "ORGANIZER" AND THE INDUC- 

 TIONS CONSIDERED AS MORPHO- 

 GENETIC FIELDS 



The main difficulty in the analysis of in- 

 duction arises from the fact that in most 

 cases both the inductors and the induced 

 material originally constitute "morpho- 

 genetic fields" which defy fvirther break- 

 down into localizable subunits. We are 

 dealing with complex dynamic systems and 

 not with an assembly of independent fixed 

 primordia. Let us briefly retrace these dif- 

 ficulties and then try to untangle them as 

 far as it seems possible at this moment. 



Field Characteristics of the "Organizer." 

 The concept of "morphogenetic fields" 

 (Spemann, '21a; Gurwitsch, '22; Weiss, '26, 

 '39) emerged from the older concept of 

 "harmonious-equipotential systems" (see 

 Driesch, '29), which stressed the regulative 

 capacities inherent in the majority of em- 

 bryonic systems. However, the doctrinal 

 formulations of Driesch barely did justice 

 to the actual conditions in the embryo (see 

 the critical discussion of Needham, '42, p. 



119). A typical example of a morphogenetic 

 field is the chorda-mesodermal area of the 

 early gastrula: any isolated part of it tends 

 to regulate into a well-proportioned axial 

 system comprising considerably more kinds 

 of tissues than would arise from this mate- 

 rial in a normal embryo; parts of this field 

 can be removed or interchanged without 

 causing abnormal development; two chorda- 

 mesoderm fields can be fused to form a sin- 

 gle axial system. This means that the 

 histological fate of any part of the field de- 

 pends upon its topographic relationship to 

 the other parts of the field. There are many 

 other examples of such regulative fields both 

 in vertebrate and invertebrate development 

 (see Huxley and De Beer, '34; Weiss, '39). 



We do not know how the chorda-meso- 

 derm field comes into existence, but we do 

 know that there are complicated determina- 

 tive inter-relationships between the constitu- 

 ents of this field leading to its segregative 

 differentiation. This poses the question of 

 whether the determinative effects within 

 the field involve principally the same physi- 

 ological mechanisms as are operative in in- 

 duction. The latter is characterized by a 

 unidirectional action of inducing upon 

 reacting tissues. In a morphogenetic field, 

 however, there seem to exist reciprocal de- 

 terminative actions which are not strictly 

 confined to close contact-relationships. Fur- 

 thermore, there was evidence that the 

 ectoderm of the gastrula, itself not an in- 

 ductor, contributes to the fixation of the 

 boundaries of the chorda-mesoderm field. 

 Contrary to the conditions in ordinary in- 

 duction, this ectodermal effect would have to 

 operate in a tangential direction and not be- 

 tween two superimposed cell layers. 



These considerations are a challenge to 

 find common principles which govern both 

 the inductions and the self-organization 

 which occur within a morphogenetic field. 



It is a peculiar property of the chorda- 

 mesoderm field ("trunk-tail organizer") and 

 of the dorsal blastoporal lip proper ("head 

 organizer") to induce adjacent tissues, par- 

 ticularly the overlying prospective ectoderm, 

 to start new trends of differentiation. This 

 capability of induction should not be in- 

 cluded in a general definition of morpho- 

 genetic fields (see Waddington, '34; Weiss, 

 '35). However, in vertebrate development, 

 these two properties — self-organization of 

 the primary inductors, and their regionally 

 specific inductive capacities — are intricately 

 associated. 



It would be entirely misleading to con- 



