Amphibians 



259 



'31a) has never been subjected to a rigorous 

 test. The observations of Yntema ('33) con- 

 cerning the distribution of otocyst compe- 

 tence in the late neurula do not substantiate 

 this concept. 



We have little information concerning the 

 specific factors responsible for regional dif- 

 ferences in competence. In the case of the 

 ear (Yntema, '50) and of the lens (Liedke, 

 '51) it seems that the head mesoderm pro- 

 vides the primary specific stimvili for ear and 

 lens differentiation which are locally differ- 

 ent. In all probability, this initial step of de- 

 termination {"Bahnimg," Vogt, '28a) involves 

 a loss of reactive potencies for other stimuli. 

 It is conceivable that in this way differentials 

 are set up, at first between head and trunk 

 epidermis in general, and then between 

 different head regions. This problem, which 

 has been touched upon already in connection 

 with the two-step "inductor-systems" (p. 

 255), awaits clarification by further system- 

 atic analysis. 



The data presented in this chapter indicate 

 that the competence of the ectoderm changes 

 with time and, in a rather involved way, 

 with the region of the embryo. Since the 

 entoderm and, to a lesser extent, the meso- 

 derm, develop rather independently of ex- 

 ternal determinative stimuli, we refrain from 

 discussing their particular age-conditioned 

 states of competence. As far as the ectoderm is 

 concerned, it is clear that for the achieve- 

 ment of its typical differentiations, two sets 

 of tempoi-ally and spatially matching, yet 

 relatively independent, mechanisms are re- 

 quired: (1) a proper distribution of inductors 

 which are in some measure specific; (2) a 

 proper stage-specific state of responsiveness 

 of the ectoderm. 



As was stated above, the genetic constitu- 

 tion determines decisively the response of a 

 given tissue to external stimuli. This point 

 has been mentioned above in connection with 

 the regional distribution of the competence 

 for lens formation. 



REGIONAL HOST INFLUENCE 



When living or dead inductors were 

 grafted into early gastrulae, results were 

 obtained which could not always be ac- 

 counted for by inductive specificity of the 

 graft but which indicated the interference 

 of regional host influences. Such effects were 

 first shown in the experiments of Spemann 

 ('31). Whereas "head organizer" would in- 

 duce brain structures in any body level of 

 the host, "trunk organizer" did not express 



its specificity when transplanted into the 

 head level, since there it induced eyes and 

 parts of brain which were fused with those 

 of the host. 



Various explanations have been offered for 

 this host influence, but before discussing 

 them, it should be pointed out that certain 

 phenomena sometimes listed under this head- 

 ing may be due to purely mechanical inter- 

 actions between transplant and host. (1) 

 The fact that the structures induced by 

 grafted blastoporal material tend to appear 

 at the same body level as the corresponding 

 host structures may be due to a deflection 

 of the invagination movements of the graft 

 by those of the host whereby both become 

 concordant. (2) Double or multiple brains 

 and sense organs can arise if the graft acts 

 as a mechanical obstacle, by splitting or 

 deranging the inductors of the host. Grafts 

 which are otherwise rather inactive can 

 produce this effect (Raven, '33; Holtfreter, 

 '34a), and it is not unlikely that some of 

 Spemann's results can be explained in this 

 way. 



The elusiveness of this problem is indicated 

 by the fact that normal trunk-tail organizer 

 (Mangold, '33b; Holtfreter, '36) as well as 

 adult organs (Chuang, '39) can induce a 

 tail in the host's head region. Obviously, a 

 clarification of this problem can only come 

 from experiments which deal with it on a 

 statistically satisfactory basis and avoid the 

 situations outlined above. Besides, an analy- 

 sis of the host infhiences would require a 

 comparison of the inductions obtained by the 

 same grafts in the whole embryo and in 

 isolated ectoderm. 



Normal head and trunk inductors express 

 their regional specificity in every case when 

 they are grafted into isolated ectoderm vesi- 

 cles (Holtfreter, '36; Okada and Hama, '43, 

 '45; Fig. 83). Comparable results were ob- 

 tained when fresh or boiled adult tissues 

 were implanted into such explants. For in- 

 stance, Chuang ('38, '39) found that mouse 

 kidney, when acting upon explanted ecto- 

 derm, would induce exclusively brain and 

 sense organs, whereas Triturus liver would 

 induce trunk and tail in addition to head 

 structures (Figs. 93-96; Table 14, p. 269). 

 Since the inductions obtained in such ex- 

 plants could be just as complex and well 

 organized as those produced by the same in- 

 ductor in whole embryos, these findings in- 

 validate the idea of some authors that 

 atypical inductors merely "evoke" a general- 

 ized strvicture such as neural tissue and that 

 an "individuation field" of the host is re- 



