390 



THE FURTHER DIFFERENTIATION OF THE 



which normally supply the limb do not grow to the place where it 

 ought to be, but to the place where it is, provided that this is not 

 more distant than two or three segments away from its normal 

 position.^ If this distance from the normal position is exceeded, the 

 limb becomes innervated by other nerves, corresponding to the 

 level of its position, which would not normally have supplied 

 a limb at all. In Amblystoma, the normal supply to the fore-limb 

 is composed of fibres from spinal nerves 3, 4 and 5, forming the 



Fig. 189 

 The attraction of outgrowing nerve-fibres towards an abnormally situated limb. 

 a, The constitution of the normal brachial plexus of an axolotl formed from spinal 

 nerves 3-5. b, The brachial plexus of an embryo in which the limb-bud was 

 moved five segments further back; the plexus is formed by spinal nerves 5-9. 

 (From Mangold, Ergehn. der Biol, in, 1928. after Detwiler.) 



brachial plexus. But the plexus can be formed from spinal nerves 

 2, 3 and 4, or 5, 6 and 7 (sometimes with the co-operation of 

 additional nerves) (fig. 189). 



The attraction which the limb exerts on the outgrowing nerve- 

 fibres is shown still more clearly by experiments in which the whole 

 of one half (the right) of the rudiment of the spinal cord of the frog 

 is removed at the neurula stage. No nerves at all grow out from the 

 right side towards the hind-limb, but fibres from the sciatic plexus 



^ Detwiler, 1920 b, 1922. 



