AMPHIBIAN NERVOUS SYSTEM 393 



muscles of the limb) is not attracted by the muscle-fibres of its 

 segmental myotomes, for the latter are already innervated whereas 

 the muscles of the limb are not. 



It is to be noted that when a limb is grafted to an abnormal 

 position, nerve-fibres are not only attracted to it, but they form in- 

 timate functional contact with its muscles. An eye or a nasal pit, 

 on the other hand, can attract the nerve-fibres to their vicinity, but 

 no more ; no intimate functional contact is established. These facts 

 have led to the view that the establishment of functional contact 

 and innervation is controlled by factors of a specific kind for each 

 type of structure, possibly chemical in nature.^ If this hypothesis 

 should turn out to be justified, then, in the outgrowth of a nerve- 

 fibre and its functional innervation of an end-organ, both non- 

 specific and specific factors would be involved. 



§7 

 We may now turn to the differentiation of the cells of the neural 

 crest. Many of these, of course, give rise to the neurons of the 

 dorsal-root ganglia, but it appears that the metamery and differ- 

 entiation of the ganglia is dependent on the presence of the seg- 

 mented myotomes. If at the tail-bud stage of Pleiirodeles the myo- 

 tomes are removed from one side of the trunk without damaging 

 the neural crest, the resulting embryo lacks spinal ganglia on the 

 operated side.^ Similarly, the spinal ganglia fail to develop norm- 

 ally in experiments in which portions of spinal cord are grafted 

 without myotomes into the flank of other ernbryos.^ Conversely, 

 the interpolation of an extra myotome as a result of grafting leads 

 to the formation of an extra spinal ganglion.^ 



In addition to giving rise to neurons, some of the cells of the 

 neural crest have been experimentally shown to produce the sheath 

 cells, which enclose the peripheral nerves. If in Amhlystoma the 

 neural crest is removed in the region of the trunk, no dorsal nerve- 

 roots or ganglia are developed: the ventral nerve-roots develop 

 normally, but have no sheaths. On the other hand, if the ventral 



1 Cajal, i9o6;Tello, 1923. 



^ Lehmann, 1927. 



^ Yamane, 1930. 



* Detwiler, 1932, 1933 b. 



