116 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. 



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 

 niiuht 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 

 \\viit 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 

 free end pushes its way through 

 the tissues surrounding it. This 



shewing a piece of spinal cord with the has not been sllOWll by Harri- 



developing motor nerve but ignoring the son ' g experiments nor COllld it 



niyotome which is in the actual embryo ., , \ , ., . 



continuous with the outer end of the nerve. pOBBlbly 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 

 i i MM vase in bulk of the body, at no period from the earliest stage 

 li.L'uml has it a free end; it is throughout connected with its end- 

 organ. 1 



In ;i word, it appears to the present \\riter thai what are 

 eoinnionly regarded as the most convincing pieees of evidenee in 

 favour of the His view are by no means eon\ ineiii-. 



Views ivMMuhling that of His in thai they also involve a.n out- 



1 Tin; actively moving pMudopodium-Ukc fcAffl \\hidi Harri^m ohser\ed ;it the 

 end id' his out^rou in^ nerve trunk .-ire l>elie\ed by (he promt \\riier i> lir nirx-n 

 'hymatoiis in their nature possibly shreds of sheath protoplasm. li i 

 feature of eiubryonie meseiichynie ihat its protoplasm slm\\ .s act i \ aim n-bo id ' ni,\ e 

 men). 



FIG. 65. Drawings taken from the same 

 preparations as those illustrated in Fig. 60, 



