148 OT<r THE HOMOLOGY Or THE CONAMO-HTPOPHTSIAL TEACT. 



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 "*. 



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

 minal cords," ganglionic or otherwise, would doubtless hold the 

 same relative situations in an abstract view of the structures, 

 irrespective of their assumed relative positions in the Annulate 

 and Yertebrate 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 alimentarj^ canal dividing 

 the fore brain from the hind brain in Annulates, and the parts of 

 the homologue of the myelencephalon (" cerebro-spinal tract " or 

 abdominal nerve-cord) become wholly on the neural aspect of the 

 body, as in Vertebrates. In both divisions the infolding of the 

 side walls completing the central canal occurs on the neural 

 side. In both the nerves arise from the neural summits of such 

 infoldings : and in both the " external skin " would pass from the 

 neural side of the groove into the central (then becoming) ciliate 

 canal. In both the haemal side of the cord would manifest an 

 excess of the " white matter ;" and this with the opposite pre- 

 dominant grey matter would present not " very nearly," but the 

 very same relative positions to the body of the animal containing 

 them (compare figs. 2 and 3). 



Of these propositions, the base or support is the homology 

 of the pineal, third ventricular, infundibular, pituitary residuary 

 modifications in the Vertebrate brain with the persistent func- 

 tional canal traversing the homologous tract in the Annulate or 

 Articulate brain t. 



* Balfour, op. cit. p. 165. 



t I have elsewhere (' Archetype of the Vertebi-ate Skeleton,' 8to, 1848, p. 2) 

 pleaded in favour of single substantive " terms " in place of " descriptive " 

 phrases," and may here cite, as synonyms of "myelencephalon" — "nervous 

 system" (p. 165), "cerebro-spinal nervous system "(p. 99), "central neryous 

 system " (p. 100), " nervous part of the brain and spinal cord " (p. 100) ; again, as 

 synonyms of " myelon " — "spinal marrow," " spinal cord," " abdominal nerve- 



