304 Experimental Zoology 



grafted upon another individual. On the other hand, the 

 limb bud of a normal individual has already received its proper 

 stimulus before its removal, hence the independent development 

 in it of the peripheral nerves. While the argument is far from 

 convincing, 1 yet the method gives promise of throwing light on a 

 very difficult and obscure point. 



There is another result of interest connected with these graft- 

 ing experiments of Braus. He finds when the bud of the fore leg 

 is grafted in another part of the body near the hind leg, for 

 example that in the majority of cases two legs develop, one 

 at first more advanced than the other, and the more advanced leg 

 alone contains nerves. In the light of some recent experiments 

 of Tornier, in which four complete hind limbs of the frog are ar- 

 tificially induced by splitting the limb buds, there can be no doubt 

 that the less advanced leg, in Braus's experiment, is a regenera- 

 tive product. It fails to receive nerves either from its twin leg, 

 of which it is a mirror figure, or from the main body of the tad- 

 pole, or at least no medullated nerves. 



Aside from the interesting questions concerned in the forma- 

 tion of the nerves the experiments of Harrison and of Braus are 

 important in showing that as complicated an organ as the leg 

 of the frog may develop in the complete absence of peripheral 

 nerves. The result shows, if true, that the self -differentiation of 

 all the tissues of the leg may take place in the entire absence of 

 connection by means of nerves with the central nervous system. 



1 The experiments of Braus have been repeated by Harrison upon frog and 

 toad embryos, the result having been briefly reported at the Toronto meeting of 

 the British Medical Association, August, 1906. Contrary to Braus, Harrison 

 finds that nerves are present, after a time, in limbs developed from buds which are 

 transplanted from "nerveless" embryos. These nerves have normal relation 

 to the other structures in the limb, at least as far as the main features are con- 

 cerned, though the finer details have not yet been studied. Furthermore, in 

 cases where two limbs have developed out of a single transplanted bud, Harrison 

 has found that nerves are present in both. Braus's results are probably to be 

 explained by the fact that he did not keep his specimens alive for a sufficient 

 length of time after the operation. Be this as it may, Harrison's experiments show 

 beyond doubt that the nerves grow from the body of the host into the trans- 

 planted appendage, and also that it is the configuration of the organs and tissues 

 in the appendage that determines the course taken by the outgrowing fibers. 



