Amphibians 



265 



(Wagner, '49). Dermal bones of the visceral 

 skeleton, which are characteristic of early 

 larvae of urodeles but absent in anuran tad- 

 poles, were formed by the mesectoderm of 

 Triturus in the anuran head in typical loca- 

 tion and at the normal time. Conversely, the 

 mesectoderm of Bombinator formed typical 

 rostral cartilages in the urodele head which 

 normally lacks these structures. These results 

 imply that in both orders the entomesodermal 

 head tissues are homodynamic in their capac- 

 ity to provide a favorable substratum for the 

 migration and final topographic localization 

 of the skeletogenous neural crest derivatives. 

 In addition, the visceral entoderm supplies an 

 agent necessary for chondrogenesis (Horstad- 

 ius and Sellman, '45) which is equally 

 effective (that is, homodynamic) in both 

 orders. Anuran mesectoderm formed tooth 

 papillae in the larval Triturus head, although 

 normally the anuran tadpoles do not form 

 teeth imtil metamorphosis. Bombinator and 

 Triturus mesectoderm are thus shown to be 

 homodynamic in their competence to stimuli 

 which induce papillae. 



Baltzer ('50a, b) has pointed out that 

 the homodynamic components of the develop- 

 mental mechanisms which are a common 

 stock of large taxonomic units represent an 

 as yet unexhausted reservoir for future evo- 

 lutionary divergencies. The specific compo- 

 nents, on the other hand, indicate where 

 genetic changes have occurred in the past. 

 The ectoderm appears to be predominantly 

 a carrier of specific factors, while the in- 

 ducing mesoderm and entoderm carry the 

 homodynamic factors, although this is by 

 no means a general rule. In several instances, 

 the evolutionary change seems to have in- 

 volved shifts in the time patterns of differen- 

 tiation, as is illustrated in the delay of 

 dermal bone and tooth formation in anurans 

 (for a discussion of these and related prob- 

 lems, see DeBeer, '51). 



ANALYSIS OF THE PHYSIOLOGICAL 

 MECHANISM OF INDUCTION 



From the preceding account rather com- 

 plete information has been gained concerning 

 the morphological phenomena of induction 

 in the amphibian embryo. Departing from 

 this level, further research proceeded to 

 examine the induction mechanism from a 

 more physiological and biochemical angle. 

 Since it is impossible in this context to re- 

 view adeouately the vast literature in this 

 field, we shall consider only those data which 

 are relevant to our topic. 



INDUCTIVE CAPACITY OF DEAD 

 EMBRYONIC TISSUES 



In 1932, a short collective report by Bautz- 

 mann, Holtfreter, Spemann, and Mangold 

 announced that organizer and medullary 

 plate retained inductive power after they 

 had been killed by heat, cold, or alcohol 

 (Fig. 99). Holtfreter showed in addition 

 that non-inductors, such as ectoderm and 

 entoderm of the gastrula, acquired induc- 

 tion capacity through these treatments. More 

 intensive studies were then devoted to the 

 effects of extreme temperatures and various 

 chemical agents on the inductive capacity of 

 embryonic tissues (Holtfreter, '33e, '34a). 

 The test method consisted largely in grafting 

 the killed tissues into the blastocoele of early 

 Triturus gastrulae. 



One of the first surprises in these experi- 

 ments was the observation that the neuraliz- 

 ing capacity of the inductors was hardly 

 affected by drying, or boiling at 100° C, 

 whereas the mesodermizing capacity disap- 

 peared rapidly through these treatments. At 

 temperatures between 100° and 150° C. the 

 neuralizing effect was likewise progressively 

 reduced to zero. 



After it had been either dried, heated, 

 frozen, or treated with acid, alcohol, or boil- 

 ing ether, the posterior part of the medvillary 

 plate plus subjacent archenteron roof which, 

 in the living state, can induce trunk and 

 tail tissues, had not only lost its mesodermiz- 

 ing activity but also changed its regional 

 specificity: it now induced brain portions 

 which could be associated with various sense 

 organs (see also Barth and Graff, '38; Okada 

 and Takaya, '42; Holtfreter, '48a). Likewise, 

 ectoderm and ventral entoderm from a gas- 

 trula and early neurula which were rendered 

 active by extreme temperatures or fat solvents 

 had a strong tendency to induce brain por- 

 tions and sometimes free lenses, but they 

 never induced mesoderm. Thus, through the 

 different killing methods, the original re- 

 gional specificity of the normal inductors 

 was abolished, and all parts of the gastrula 

 or neurula became inductors of cephalic 

 structvires. Also inductive were all cyto- 

 plasmic regions and the nucleus of the 

 coagulated ovum (Holtfreter, '34a; Wadding- 

 ton, '38b). 



In order to disarm the possible criticism 

 that the high degree of organization found in 

 the induced structures might have been due 

 to an activation of the host's determination 

 fields (Weiss, '35; Woerdeman, '36), the 

 host influence was excluded in the following 



