28 



CONAUIO-IIYPOPHYSIAL TRACT. 



whicli may be assumed by the living animal, but the relative 

 positions to such body of the central parts of the nervous and 

 vascular systems, which relations I have expressed by the terms 

 "neural" and "hfemal." The convenience of these terms 

 or signs is exemplified by the trouble, not to say perplexity, 

 which arises when characters, or developmental phenomena, 

 as of the umbilical -vesicle, repeated in Vertebrates and Arti- 

 culates, 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 the 

 formation 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 anthropo- 

 tomy, 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 anthropotomically 

 "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 con- 

 sists 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." . . . . " 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, 



