HOMOLOGY. 



2,9 



above and belowwliich the two columns of the nervous system 

 united, we should have in the transformed nervous cord an 

 organ strongly resembhng the spinal cord of Vertebrates"*. 

 But a resemblance, hovv^ever strong, between the two parts 

 or organs is not, of itself, a ground for predicating homology. 

 For, as the accomplished developmentalist proceeds, " 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 con- 

 tinuous 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 A'ertebrate 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" f. 



Parts of the important organs " spinal marrow " and " ab- 

 dominal cords," ganglionic or otherwise, would doubtless hold 

 the same relative situations in an abstract view of the struc- 

 tures, irrespective of their assumed relative positions in the 

 Annulate and Vertebrate bodies ; but in relation to the 

 accepted position of the nerve-centres in the two groups they 

 would hold opposite relative situations in and to the body ; 

 the extreme summits of the primitive folds giving origin to 

 the nerves would be dorsal in the Vertebrate, and ventral in 

 the Annulate modifications of the animal structures. 



Obliterate the mouth and part of the alimentary canal 

 dividing the fore brain from the hind brain in Annulates, and 



* Balfour, op. cit. p, 165. 

 t Ibid. 



