Amphibians 



263 



structures into their regional fields. We recall 

 the observation that the inductors in the 

 archenteron roof represent overlapping fields 

 which reach far into the lateral parts of the 

 embryo. The normally imexpressed activity 

 of the periphery of these fields may inter- 

 vene and superimpose its regional character 

 istics upon the inductions of the graft. Such 

 an auxiliary effect seems to account for the 

 fact that the closer a graft lies to the dorsal 

 host mesoderm, the more pronounced is its 

 neural induction (Raven, '33). 



There is also the "bridge phenomenon": 

 An induced nem-al plate lying close to that 

 of the host usually links up with the latter, 

 suggesting that sub-threshold inductive fac- 

 tors of the lateral mesoderm can add up with 

 those of the graft to induce the neural 

 bridge. The most impressive indication of 

 such an auxiliary host effect was found by 

 Pasteels ('47a,b, '49a) who produced second- 

 ary axial systems by centrifuging blastula 

 and gastrula stages. The secondary formations 

 did not attain perfect differentiation unless 

 they appeared linked up with the primary 

 axial system {"contagion" Pasteels). 



The interpretation of host effects in terms 

 of regional fields meets with the difficulty 

 that the peaks of frequency of the induced 

 organs are not always at the levels of the 

 respective host organs, where one would ex- 

 pect them. 



Obviously, the term 'host influence" com- 

 prises a group of heterogeneous factors, and 

 their analysis offers serious obstacles. In a 

 given instance, it is difficult to decide 

 whether the graft suppresses the activities of 

 the host inductors, or whether it displaces the 

 host inductors or cooperates with them. 

 Furthermore, the regional specificity of the 

 grafts themselves is somewhat variable and 

 can be expressed only in statistical terms. 



HETEROPLASTIC AND XENOPLASTIC 

 TRANSPLANTATIONS * 



Born (1897) was the first to accomplish 

 the fusion of parts of amphibian embryos 

 belonging to different species and genera. 

 This new method proved to be of great value 

 in the analysis of some basic problems such 

 as the interactions of genetic and environ- 

 mental factors in growth (reviewed by Har- 

 rison, '35; Twitty, '40), the formation of pig- 

 ment patterns (see Twitty, '42, '45), and 



* "Heteroplastic" usually refers to tissue combi- 

 nations between different species of the same genus, 

 and "xenoplastic" (Geinitz, '25b) to those between 

 more distantly related forms. 



embryonic induction. The following discus- 

 sion is concerned only with the last-men- 

 tioned problem. 



The combination of inductors and reacting 

 tissues differing in hereditary characteristics, 

 such as pigmentation, or size of cells and 

 nuclei, allows for an exact distinction be- 

 tween transplanted and host tissues. There- 

 fore, this method has become an irreplaceable 

 tool for the finer analysis of induction. Ex- 

 periments of this type have established 

 several fvmdamental principles: 



1. Induction operates across the borders 

 of species and even orders. This has been 

 shown in a variety of experiments. For in- 

 stance, the optic vesicle of a urodele embryo 

 can induce a lens in competent epidermis of 

 other urodele species or of anurans, and the 

 chorda-mesoderm can induce a secondary 

 embryo in the gastrula of a foreign species 

 or order. It follows that the inductive agents 

 are not species-specific. 



2. The responses to inductive stimuli are in 

 accordance with the genetic potentialities of 

 the reacting material; they are species- 

 specific with respect to form, growth rate, 

 and other tissue characters. For instance, a 

 balancer induced by head structures of 

 Triturus cristatus in transplanted prospective 

 belly ectoderm of T. taeniatus is a typical 

 taeniatus-balancer (Rotmann, '35a; see Fig. 

 106). The genetic constitution is thus recog- 

 nized as one of the most important limiting 

 factors for the competence of the reacting 

 tissue. 



3. The larvae of urodeles and anurans dif- 

 fer in various order-specific structures: The 

 urodeles are equipped with a pair of bal- 

 ancers on the ventrolateral side of the head, 

 whereas the anurans have a pair of adhesive 

 glands ("suckers") in a more ventral position. 

 The larvae of urodeles possess dentine teeth, 

 whereas the anuran tadpoles do not develop 

 them until metamorphosis; instead, tad- 

 poles are equipped with horny denticles 

 which are not homologous with the former. 

 There are conspicuous differences in the 

 shape, number and topographic relations of 

 the cartilages of the visceral skeleton, in body 

 pigmentation and other characters. The in- 

 duction of these structures which are diver- 

 gent in the two orders has been analyzed by 

 means of xenoplastic transplantations. It was 

 found that the inductors are responsible for 

 the regional specificity of the induced organs, 

 but the pattern and structural characters of 

 the latter are determined by the inherent 

 properties of the reacting material. The fol- 

 lowing experiments illustrate this point. 



