272 FIELDS AND GRADIENTS 



limb grafted on to the base of a tail actually produces a little super- 

 numerary tail (fig. 127). 



The undetermined stage of the regeneration-bud is of limited 

 duration. Whereas a bud of hemispherical form is still undeter- 

 mined, by the time a markedly conical shape has been attained, the 

 bud is determined, and if grafted elsewhere will now continue to 

 differentiate in accordance with its place of origin instead of in 

 accordance with its new situation. In this respect regenerated tissue 

 behaves just as do the various regions of the amphibian egg, which 

 also pass from a plastic to a determined phase.^ 



Fig. 127 

 Lack of determination in early regeneration-buds. Triton larva showing a tail 

 (against a square of white paper) developed from an early limb regeneration-bud 

 grafted into the tail-field. (From Guy^not, Rev. Suisse de Zool. xxxiv, 1927.) 



The success of the converse experiment in which the early re- 

 generation-bud of a tail is grafted on to the stump of an amputated 

 hind-limb, or into the fore-limb field, close to the base of the (un- 

 operated) host-limb, and then produces a limb,^ has also been 

 reported, but this, though highly probable, cannot be regarded as 

 conclusively proved^ (fig. 128). 



^ Guy^not, 1927; Guyenot and Ponse, 1930. 



" Weiss, 1927 B. ■ 



^ The experiment was done with regeneration-bud and host belonging to the 

 same species, and it is difficult therefore to be absolutely certain that the limb 

 developed from the grafted cells. Further, as pointed out by Guyenot, the graft 

 may have come under the influence of the endings of the brachial nerve, which 

 are known to be able to produce the formation of a limb (see p. 362). However, 

 the presumption is that Weiss' interpretation is correct. 



