AMPHIBIAN NERVOUS SYSTEM 391 



of the left side turn back across the mid-Hne, and innervate the right 

 hind-leg. In general, it appears that the pattern of the plexus 

 formed is largely independent of the amount, origin, and direction 

 of ingrowth of the immigrant nerve-fibres, and is determined by 

 factors intrinsic to the limb.^ 



In yet other experiments, on newt larvae in which a fore-limb is 

 grafted into the flank of the body close to an intact hind-limb, and 

 the nerve to the hind-limb is severed, the nerve regenerates and in- 

 nervates both autochthonous hind-limb and grafted fore-limb. The 

 actual details of innervation vary in each experiment. It is possible 

 for the grafted fore-limb to be completely innervated by branches 

 of the third lumbar nerve, which normally supplies only the ad- 

 ductors of the femur and the flexors of the knee. This shows that 

 nerves may be attracted towards and innervate muscles different 

 from those which they normally supply.^ The same conclusion 

 emerges from experiments on Amblystoma in which a limb-rudi- 

 ment is partially removed at the early tail-bud stage, and grafted 

 back into the same embryo at a distance of four segments posterior 

 to the normal position. From the remainder of the rudiment in the 

 normal position a limb is also formed, so that the embryo has two 

 fore-limbs on the same side, and the nerves of the brachial plexus 

 may be supplied to both.^ In these cases, an additional point of 

 interest is the fact that both limbs show simultaneous movement 

 of homologous muscles, although the actual nerve-fibres which 

 innervate them may be quite different, and their distribution varies 

 in each individual case.^ 



The attraction which is exerted by a limb on a growing nerve is 

 even less specific than would appear from the experiments just 

 mentioned, for it is also exerted by an eye or a nasal pit, grafted 

 on to the side of the body of a larva (in Amblystoma) , after removal of 

 the limb-rudiment.^ In these cases the nerve-fibres which would 

 normally have innervated the limb grow towards the eye or the 

 nasal pit as the case may be, and end in the tissue immediately 

 surrounding it. 



^ Hamburger, 1927, 1929. ^ Weiss, 1924 A. ^ Detwiler, 1925 A. 



* Experiments of this type have led to the so-called resonance theory of nerve 

 action. See Weiss, 1924 a, 1928 ; Versluys, 1927, 1928 ; Detwiler, 1926 c, 1930 B, 

 c; Detwiler and Carpenter, 1929; Detwiler and McKennon, 1930. 



^ Detwiler, 1927 C 



