IQ2 DEVELOPMENT OF THE SPINAL NERVES 



two lateral columns of the nervous system, became converted 

 into a canal, above and below which the two columns of the 

 nervous system united, we should have in the transformed 

 nervous cord an organ stronglyHtesembling the spinal cord of 

 Vertebrates. 



This resemblance would even extend beyond mere external 

 form. Let the ventral nervous cord of the common earthworm, 

 Lumbricus agricola, be used for comparison 1 , a transverse sec- 

 tion of which is represented by Leydig 2 and Claparede. In this 

 we find that on the ventral surface (the Annelidan ventral 

 surface) of the nervous cord the ganglion-cells (grey matter) () 

 are situated, and on the dorsal side the nerve-fibres or white 

 matter (//). If the folding that I have supposed were to take 

 place, the grey and white matters would have very nearly the 

 relative situations which they have in the Vertebrate spinal cord. 



The grey matter would be situated in the interior and 

 surround the epithelium of the central canal, and the white 

 matter would nearly surround the grey and form the anterior 

 white commissure. The nerves would then arise, not from the 

 sides of the nervous cord as in existing Vertebrates, but from 

 its extreme ventral summit. 



One 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 arise in the exact manner which 

 might have been anticipated were the spinal cord due to such a 

 folding as I have suggested. The argument from the nerves 

 becomes the stronger, from the great peculiarity in the position 

 of the outgrowth, 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 is to be explained as a rem- 

 nant of the ciliation of the external skin now found amongst 

 many of the lower Annelids. 



1 The nervous cords of other Annelids resemble that of Lumbricus in the relations 

 of the ganglion-cells of the nerve-fibres. 



2 Tafcln zur vcrglcichcndcn Anatomic, Taf. iii. fig. 8. 



