116 INDUCTION AND ORGANISATION 



and somites. Many experiments by Lehmann (1926-35), Holt- 

 freter (1933), and others have demonstrated that defects in 

 the archenteron roof cause characteristic abnormalities in the 

 brain and spinal cord. Further, it was shown that the chorda- 

 mesoderm retains its inductive powers for a long time. Pieces 

 of notochord or somites from older embryos (neurula or tail- 

 bud stage) have been investigated by the "insertion method". 

 They proved still to be able to induce a neural plate in the 

 ectoderm of the gastrula (Bautzmann, 1928; Holtfreter, 1933). 

 In contrast, the reactivity of the ectoderm to stimuli inducing 

 a neural plate was found to be restricted to a certain stage 

 only^). Once the germ has reached the neurula stage, the 

 reactivity has disappeared. This is the reason why the secondary 

 neural plate nearly always becomes visible simultaneously with 

 the primary neural plate, even though the inducing stimulus 

 has had an opportunity to exert its influence long before that. 



The inductive activity is not species-specific. In the original 

 work of Spemann and H. Mangold an organiser of Triton 

 cristatus was already found to be active after heteroplastic 

 transplantation into T. taeniatus. Moreover, induction was 

 found to take place when two more distant species were com- 

 bined (so-called xenoplastic transplantation). In Geinitz' (1925) 

 experiments, marginal zone material not only from axolotl and 

 Pleurodeles, but even from frogs (Rana fusca and R. esculenta), 

 and toads (Bomhinator) induced neural plates in the ectoderm 

 of Triton. 



Even greater interest was aroused by the discovery that the 

 power of inducing a neural plate was possessed, not only by 

 the roof of the archenteron, but also by other tissues, which 

 are not in normal development involved in the formation of 

 the neural plate. This was demonstrated by bringing such ab- 

 normal inductors into immediate contact with overlying gastrula- 

 ectoderm by means of the "insertion method" mentioned above. 

 Inductive powers were found not only in the neural plate itself, 

 and in brain tissue of older larvae (Mangold and Spemann, 

 1927), but in material of the larval limb primordia as well 



1) Nowadays such a reactivity is often called "competence". 



