VERTEBRATE AND ANNELIDAN NERVOUS SYSTEMS. 387 



The obvious difficulty, already alluded to, of understanding 

 how it is, according to the generally accepted mode of develop- 

 ment of the spinal nerves, that precisely similar nerve-cells and 

 nerves should arise in structures which have such different 

 origins as the central nervous system and the spinal nerves, is 

 completely removed if my statements on the development of the 

 nerves in Elasmobranch represent the truth. 



One point brought out in my investigations appears to me 

 to have bearings upon the origin of the central canal of the 

 vertebrate nervous system, and in consequence upon the origin 

 of the vertebrate nervous system itself. This point is, that the 

 posterior nerve-rudiments make their first appearance at the 

 extreme dorsal summit of the spinal cord. The transverse 

 section of the ventral nervous cord of an ordinary segmented 

 Annelid consists of two symmetrical halves placed side by side. 

 If by a mechanical folding the two lateral halves of the nervous 

 cord became bent towards each other, while into the groove 

 between the two the external skin became pushed, we should 

 have an approximation to the vertebrate nervous system. Such 

 a folding as this might take place to give extra rigidity to the 

 body in the absence of a vertebral column. 



If this folding were then completed in such a way that the 

 groove, lined by external skin and situated between the 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 strongly resembling the spinal cord of Vertebrates. 



It is well known that the nerve-cells are always situated on 

 the ventral side of the abdominal nerve-cord of Annelids, either 

 as a continuous layer, or in the form of two, or more usually, 

 three bands. The dorsal side of the cord is composed of nerve- 

 fibres or white matter. If the folding I have supposed were to 

 take place in the Annelid nervous cord, the grey and white 

 matters would have very nearly the same relative situations as 

 they have in the Vertebrate spinal cord. The grey matter would 

 be situated in the interior and line the central canal, and the 

 white matter would nearly surround the grey. The nerves 

 would then arise, not from the sides of the nervous cord as in 

 existing Annelids, but from its extreme ventral summit One 



