n NERVE DEVELOPMENT 115 



implanted limb in such a case (Fig. 64) is situated in a region inner- 

 vated by the facial nerve and the study of sections showed that the 

 nerves in the implanted limb were continuous centrally with 

 branches of the facial nerve. 



If we attempt to interpret this experiment on the outgrowth view 

 we find ourselves compelled to admit that the facial fibres concerned 

 made the serious mistake of growing into a limb rudiment and then 

 continued on their mistaken course until finally they established the 

 muscular connexions normal for the nerves of such a limb. Braus 

 repeated this type of experiment in a number of cases and there 

 appears to be no question as to the accuracy of his observations. If 

 accurate, however, they provide a formidable, if not unsurmountable, 

 difficulty for the outgrowth view — a diffi- 

 culty which is by no means got rid of by 

 the suggestion (Harrison, 1908) that after 

 arriving in the limb the nerves are " merely 

 guided in their growth by the structures 

 present in the transplanted part." 



A similar difficulty is seen in post- 

 embryonic nerve -development in the fact 

 well known to surgeons that functional 

 continuity can be established between the 

 cut central stump of one nerve {e.g. spinal 

 accessory) and the severed peripheral portion fig. 64.— Young Toad (Bom- 



of another (e.g. facial). binator) on which an addi- 



And so again in the development of ££>£££?<$* 

 anastomoses between peripheral nerves such Braus, 1905.) 

 as the well-known " dialyneury " of Gastero- 



pod molluscs, or the short circuiting of the left pulmonary nerve over 

 the dorsal side of the oesophagus which has come about in the evolu- 

 tion of the Crossopterygians and Lung-fishes. 



All such cases present great if not insuperable difficulties to the 

 His view. 



Again much of the evidence which is brought to the support of 

 the His view is seen when looked at critically to be less convincing 

 than it appears to be at first sight. Thus for example with the 

 experiments of Harrison already described, which are regarded by 

 their author as settling the whole question. Their true value will 

 become more apparent if we bring Harrison's results into correlation 

 with the results described above for Lepidosiren. 



As has already been shown, in this animal the motor nerve-trunk 

 is represented at an early stage by a bridge of soft fragile protoplasm. 

 These bridges require a very favourable object and very careful 

 technique for their detection, and it is clear that one could not 

 expect to see them in comparatively coarse preparations made by 

 excising a piece of living unfixed spinal cord. There is therefore no 

 guarantee that such protoplasmic nerve-rudiments were not already 

 present in the pieces of spinal cord investigated by Harrison. 



