264 



Embryogenesis: Progressive Differentiation 



When belly ectoderm of an anuran gas- 

 trula is transplanted to the prospective 

 head region of a urodele gastrula, it is in- 

 duced to form suckers on the ventral side of 

 the head (Spemann and Schotte, '32) and 

 horny denticles in the mouth region (Holt- 

 freter, '35b; Schmidt, '37; Spemann, '38; see 

 Fig. 97). In the reciprocal experiment, head 

 structures of the frog or toad induced bal- 

 ancers (Fig. 98) and dentine teeth in the pros- 

 pective neural ectoderm of a salamander 



evolution. This aspect has been stressed 

 particularly by Baltzer and his associates, in 

 their extensive transplantation experiments 

 (Baltzer, '41, '50a,b; Andres, '49; Wagner, 

 '49; Roth, '50). 



It seems that when structural divergencies 

 originated in the course of evolution, the 

 vmderlying inductive mechanisms were re- 

 tained, in part, and they have remained a 

 common property of both urodeles and 

 anurans. The term "homodynamic" was pro- 



Fig. 97. Fig. 98. 



Fig. 97. Xenoplastic substitution of prospective ventral head ectoderm of Triturus by belly ectoderm of 

 Rana esculenta, in early gastrula stage. The urodele head structures have induced mouth implements of 

 anuran type: horny "teeth" and suckers. (Fi-om Spemann, '38, after Schotte.) 



Fig. 98. Xenoplastic substitution of prospective ventral head ectoderm of the toad (Bombinator) by belly 

 ectoderm of Triturus taeniatus, in early gastrula stage. The anuran head structures have induced a urodele 

 balancer. (From Spemann, '38, after Botmann.) 



(Holtfreter, '35a; Rotmann, '35b). The re- 

 spective inductors are regionally specific 

 since they induce head implements, but they 

 .are at the same time svifficiently general to 

 call forth the formation of structures which 

 do not occur in the genetic repertory of the 

 inducing host. Spemann and Schotte ('32; 

 see also Spemann, '38) used the term "gen- 

 eral situation stimvilus" in describing this 

 condition. The explantation experiments of 

 Holtfreter ('36) have demonstrated that re- 

 gionally specific structures can be induced 

 xenoplastically, outside of a whole embryo. 

 All these results brought sharply into focus 

 the "release" character of the inductive mech- 

 anism, and the important role which the self- 

 organizing capacity as well as the genetic 

 constitution of the reacting tissue play in the 

 inductive process. 



The induction of organs which urodeles 

 and anurans do not share has interesting 

 implications for problems of homology and 



posed for factors which are equivalent in 

 different taxonomic groups, and the term 

 "specific" for factors in which they differ 

 (Baltzer, '50b). For instance, the cephalic 

 inductors of balancers and suckers would 

 be homodynamic, while the competence of 

 the ectoderm would represent the specific 

 factors. The same situation was revealed in 

 exchange transplantation between T. taeni- 

 atus, which is equipped with balancers, and 

 the axolotl, which lacks them. It was found 

 that both possess the balancer inductors but 

 that the axolotl ectoderm has lost the cap- 

 acity to respond to them (Mangold, '31b). 

 The following example may illustrate the 

 point that inductive agents as well as com- 

 petences can be homodynamic. 



When anterior neural crest was exchanged 

 between neurulae of T. alpestris and the toad, 

 Bombinator pachypus, its derivatives partici- 

 pated in the formation and induction of nu- 

 merous structures in the foreign head 



