116 EMBRYOLOGY OF THE LOWER VERTEBRATES oh. 



Let it be assumed that such an experiment is repeated . upon 

 Lepidosiren with a small piece of spinal cord rudiment with the 

 protoplasmic bridge attached to it (Fig. 65, A). The piece Of spinal 

 cord is well supplied with food material in the form of yolk and, if 

 kept under suitable conditions, it would go on developing. So also 

 might the protoplasmic bridge, for every one agrees that the metabolic 

 control of the motor nerve is exercised by the central ganglion-cell 

 nuclei within the spinal cord. If this happened and the process 

 went on quite normally we should get in succession stages such 



as those shown in B and C of 

 Fig. 65. 



Now these would be inter- 

 preted by Harrison presumably 

 as demonstrating the outgrowth 

 view, whereas all that they really 

 show is' that, given suitable con- 

 ditions, the motor nerve increases 

 in length — a fact which of course 

 is obvious. What is needed as a 

 demonstration of the His view 

 is not merely to show that a 

 nerve -trunk increases in length 

 but to show (1) that it normally 

 has a free end and (2) that it 

 grows within the body at a 

 greater rate than the tissues in 

 which it is embedded, so that 

 there is brought about a differ- 

 ential movement in which the 

 ,,„„-. A , „ 1V free end pushes its way through 



Fig. 65. — Drawings taken from the same . . A ,. •'. m .°. 



preparations as those illustrated in F lg . 60, ™e tlSSlieS Surrounding it. ThlS 



showing a piece of spinal cord with the has not been shown by Harri- 



developing motor nerve but ignoring the son > s experiments nor could it 

 myotome which is m the actual embryo ., -, \ , , , , . 



continuous with the outer end of the nerve, possibly be shown by this type 



of experiment. In Lepidosiren 

 the study of sections shows as has already been pointed out that, 

 although the motor nerve-trunk grows actively in length with the 

 increase in bulk of the body, at no period from the earliest stage 

 figured has it a free end ; it is throughout connected with its end- 

 organ. 1 



In a word, it appears to the present writer that what are 

 commonly regarded as the most convincing pieces of evidence in 

 favour of the His view are by no means convincing. 



Views resembling that of His in that they also involve an out- 



1 The actively moving pseudopodium-like tags which Harrison observed at the 

 end of his outgrowing nerve-trunk are believed by the present writer to be mesen- 

 chymatous in their nature — possibly shreds of sheath protoplasm. It is a general 

 feature of embryonic mesenchyme that its protoplasm shows active amoeboid move- 

 ment. 



