. Analysis of development. Embryology; Embryology. 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, '
. Analysis of development. Embryology; Embryology. 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
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