OF THE CONAEIO-HTPOPHTSIAL TRACT. 147 



fied by the trouble, not to say perplexity, wliicli arises wben 

 characters, or developmental phenomena, repeated in Vertebrates 

 and Articulates, are endeavoured to be expressed or expounded 

 on the " dorsal " and " ventral " homological hypothesis. 



Balfour, for example, in his keen and accurate views of the 

 primary growths of the myelon, in Elasmobranchs, traces tlie for- 

 mation of the central cavity by the " dorsal" folding of the lateral 

 halves of the primitive open canal, which includes the grey matter 

 and carries in also a fold, now become the lining of the cavity, of 

 the embryonal ciliate epiderm. 



The primal nerve-roots are, or are attached to, free margins of 

 the dorsal folds, and become the " dorsal," or, in anthropotomy, 

 the " posterior " roots of the spinal nerves. The white matter of 

 the myelon becomes external and lies in greater proportion along 

 the under, or ventral, or antliropotomically " anterior," part, than 

 on the " dorsal " part of the myelon. 



Now comes the difficulty arising from the non-appreciation of 

 the homology of the conario-hypophysial infundibular tract with 

 the annulose gullet. " 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 fold- 

 ing 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." .... " If this folding were then com- 

 pleted in such a way that the groove, lined by external skin and 

 situated between the two lateral columns of the nervous system, be- 

 came 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 " *. But a resemblance, however 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 

 continuous layer, or in the form of two, or more usually, three 

 bands. The dorsal side of the cordis 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 tlie 

 * Balfour, ojp. cit. p. 165. 



