388 DEVELOPMENT OF ELASMOBRANCH FISHES. 



of the most striking features which I have brought to light with 

 reference to the development of the posterior roots, is the fact 

 of their growing out from the extreme dorsal summit of the 

 neural canal, a position analogous to the ventral summit of the 

 Annelidan nervous cord. Thus the posterior roots of the nerves 

 in Elasmobranchs 1 arise, in the exact manner which might have 

 been anticipated, were the spinal canal due to such a folding as 

 I have suggested. 



The argument from the position of the outgrowth of nerves 

 becomes the more striking from its great peculiarity, and forms 

 a feature which would be most perplexing without some such 

 explanation as I have proposed. The central epithelium of the 

 neural canal, according to this view, represents the external 

 skin, and its ciliation in certain cases may, perhaps, be ex- 

 plained as a remnant of the ciliation of the external skin still 

 found amongst many of the lower Annelids. 



I have employed the comparison of the Vertebrate and 

 Annelidan nervous cords, not so much to prove a genetic rela- 

 tion between the two, as to shew the a priori possibility of the 

 formation of a spinal cord, and the a posteriori evidence we 

 have of the vertebrate canal having been formed in the way 

 indicated. I have not made use of what is really my strongest 

 argument, viz. that the embryological mode of formation of the 

 spinal canal by a folding in of the external epiblast is the very 

 method by which I supposed the spinal canal to have been 

 formed in the ancestors of Vertebrates. My object has been to 

 suggest a meaning for the peculiar primitive position of the 

 posterior roots, rather than to attempt to explain in full the 

 origin of the spinal canal. 



Although the homologies between the Vertebrate and the 

 Annelidan nervous systems are not necessarily involved in the 

 questions which arise with reference to the formation of the 

 spinal canal, they have nevertheless considerable bearings on it. 



Two views have recently been put forward on this subject. 



1 There are strong reasons for regarding the posterior roots as the primitive ones. 

 These are spoken of later, but I may state that they depend : 



1 i) On the fact that only posterior roots exist in the brain. 



(2) That only posterior roots exist in Amphioxus. 



(.',) That the posterior roots develop at an earlier period than the anterior. 



