364 GRADIENT-FIELDS IN POST-EMBRYONIC LIFE 



A remarkable effect sometimes occurs after implantation of a 

 foreign limb-bud in Urodele larvae. The grafted limb may de- 

 generate, but its presence may stimulate the host-tissue to pro- 

 liferate and replace the grafted tissue. This was proved by grafting 

 haploid limb-buds on to diploid larvae. After a time all the haploid 

 tissue had been replaced by diploid: the formation of a super- 

 numerary limb by the host had been induced.^ In three cases it 

 appears that a grafted fore-limb which degenerated after trans- 

 plantation into the hind-limb held was replaced by host-tissue 

 which then differentiated into a hind-lim.b. This recalls the move- 

 ments of cells induced by grafts in Hydra, etc. (p. 301). 



Normally, however, the regenerate is formed definitely from the 

 remainder of the organ ; this is shown in cases where a haploid arm 

 has been grafted on to a diploid body in Triton, and then the graft 

 is cut through: the regenerate is entirely haploid. ^ Similar results 

 are found with the regeneration of Triton limbs grafted hetero- 

 plastically on to Salamanders. 



It is important to note that the morphogenetic properties of the 

 regional field itself, once they have been determined, are not in- 

 fluenced by position relative to the whole organism. For instance, 

 in Salamander larvae, fore-limbs grafted into the hind-limb field, 

 and then cut through, produce fore-limb structures in regeneration, 

 and vice versa for hind-limbs grafted into the fore-limb field and 

 made to regenerate. A portion of the determined field has here been 

 transplanted, and continues to produce structures of its proper type 

 irrespective of its position. - 



Only an extremely small portion of a determined field is needed 

 to determine the character of the structures regenerated. If a limb 

 regeneration-bud, in the stage in which it is still undetermined, 

 together with a small portion of stump, be grafted into a foreign 

 field, it will regenerate in accordance with the character of the 

 stump, not in accordance with the character of the new field as 

 would have happened if it had been grafted alone ^ (see p. 271).* 



^ G. Hertwig, 1927. ^ Weiss, 1924 b. ^ Milojewic, 1924. 



'^ The existence of sharply delimited fields diflfering in their histological and 

 physiological properties is also known from studies on Anuran metamorphosis 

 (see p. 427) and from work on bio-electric phenomena in the regions of the 

 mammalian brain (Kornmiiller, 1933). 



