Amphibians 



257 



mediation of several generations of properly 

 distributed inductors which succeed each 

 other and may combine in various ways to 

 act as coordinated determinative systems. 

 This chain of indvictive processes continues 

 to operate beyond embryonic stages, involv- 

 ing the determination of the gonads, the 

 mesonephros and various morphogenetic proc- 

 esses which occur during the metamorphosis 

 of the amphibian larvae. Thus the principle 

 of induction is of great importance for the 

 organization of the amphibian body and, 

 probably, of all other vertebrates. 



COMPETENCE 



Some districts of the early amphibian 

 embryo, particularly the entodermal ones, 

 do not seem to require specific extei^nal stim- 

 uli for their normal differentiation, while 

 the mesodermal ones do require them to some 

 extent. This dependence is most pronounced 

 in the prospective ectodermal districts whose 

 typical differentiations do not arise in the 

 absence of exogenous inductive factors. On 

 the other hand, it has been found that the 

 differential fate of the ectoderm depends not 

 only upon regional differences of the in- 

 ductors but just as much upon a proper state 

 of ectodermal responsiveness which varies 

 with the developmental stage. The physio- 

 logical state of a tissue which permits it to 

 react in a morphogenetically specific way 

 to determinative stimuli has been termed 

 "Reaktionsfdhigkeit" (Mangold, '29a), "Reak- 

 tionsbereitschaft" (Machemer, '32) or "com- 

 petence" (Waddington, '32, '40). Like the 

 capacity of certain cell groups to differenti- 

 ate normally without specific external stim- 

 uli, competence is of course primarily 

 determined by genetic factors. The term 

 "competence" is useful if one wishes to dis- 

 tinguish between cell-inherent and exoge- 

 nous factors of cell determination. 



In using this term, it is necessary to specify 

 embryonic area, stage of development, and 

 the differentiation process to which compe- 

 tence refers. We can define competence only 

 in terms of the products of differentiation 

 because the inner conditions which permit 

 an embryonic area to pursue a certain differ- 

 entiation when properly stimulated can be 

 grasped today only in a very hypothetical 

 way. It would be advisable to confine the 

 term to a limited phase and not to the termi- 

 nal steps of differentiation. As an illustration 

 of some fundamental aspects of competence, 

 data collected on the ectoderm have been 

 selected for discussion. 



TIME PATTERN OF COMPETENCE 



It is well known that the ectoderm from 

 early gastrulae can respond to different in- 

 ductive stimuli by forming any one of a 

 number of different ecto- or mesodermal 

 derivatives (p. 241). These and other data 

 refute the notion of a dichotomy of progres- 

 sive differentiation (Lillie, '27), according to 

 which only one of two directions of differen- 

 tiation is open to an embryonic area at a 

 given stage. 



With progressive development, the compe- 

 tence to form a diversity of structures be- 

 comes as a rule gradually restricted. Neural 

 induction of the ectoderm occurs normally 

 in advanced gastrula stages. Spemann ('18), 

 Mangold ('29a), Lehmann ('28, '29), and 

 others have shown that the competence of 

 the prospective epidermis for neuralization 

 is lost shortly after this event has taken place. 

 In special studies of this problem, Machemer 

 ('32), using urodeles, and Schechtman ('38a), 

 using Hyla, transplanted the upper blasto- 

 poral lip of early gastrulae under ventral 

 and flank epidermis of various stages. They 

 found that the capacity for neuralization 

 decreases markedly in the late gastrula stage, 

 and is completely absent in the early neurula 

 ectoderm. Raunich ('42b), using adult liver 

 as an inductor, obtained similar results. 



In a different set-up, Holtfreter ('38a) 

 removed pieces of prospective ectoderm from 

 early Triturus gastrulae, reared them in 

 standard solution for varying lengths of time, 

 and then implanted them homo- or xeno- 

 plastically into the dorsolateral regions of 

 neurulae where a variety of inductors were 

 known to be present. It was found that 

 competence changes with time in both a 

 quantitative and a qualitative sense. With 

 increasing age, the grafted ectoderm forms 

 increasingly smaller and at the same time 

 less complex neural structures. Furthermore, 

 new directly induced structures arise. Older 

 ectoderm corresponding to the neurula stage 

 forms only tissues which normally arise 

 from the placodes and neural crest, until 

 finally it becomes entirely refractory to in- 

 ductive stimuli. The experiments indicate 

 that the gradual change and final loss of 

 competence is due to an autonomous process 

 of ageing within the ectoderm (see also 

 Waddington, '36b). 



It is not yet possible to state at which stage 

 the ectoderm acquires its responsiveness to 

 inductive stimuli. Mangold ('26, '29a) and 

 many subsequent workers observed that 

 when living or dead inductors are grafted into 



