GRADIENT-FIELDS IN POST-EMBRYONIC LIFE 



363 



out. Each field occupies only a certain definite zone surrounding 

 the structure to which it gave rise during development. If nerves 

 are deflected to "frontier" regions between the fields, mixed 

 structures or chimaeras are produced, partaking of the nature of 

 both fields. 1 



From various lines of evidence, it appears that the action of the 

 deflected nerves in these experiments is in no way specific, but 

 merely trophic. What the nerve does is to stimulate proliferation : 

 the type of structure proliferated is a function of the specific field. 



A B 



Fig. 175 

 Effects of deflected nerves ending freely in fields. A, In the limb-field, leading to 

 the formation of a limb. B, In the dorsal-crest field, producing dorsal crest.- 

 C, In the tail-field, giving rise to extra tail. (From Guyenot, Rev. Suisse de Zool. 

 XXXIV, 1927.) 



This view is confirmed by other work, carried out on non-breeding 

 newts, in which a fine silk ligature was tied tightly round the body, 

 passing over the amputated stumps of the hind-limbs. This was 

 done in order to produce a mechanical division of the limb 

 regeneration-buds. In addition to succeeding in this object it 

 caused an unexpected effect in promoting a local proliferation on 

 the mid-dorsal line, which developed into a typically crest-like 

 structure^. This occurred whether the ligature was superficial, or 

 was passed through below the surface in the dorsal region. (See 

 also Chap, xiii, p. 430.) 



1 Locatelli, 1925; Bovet, 1930. " Milojewic, Grbic and Vlatkovid, 1926. 



