MYELENCEPHALON OF FISHES. 



269 



and tapering to both ends, but more gradually to the hinder 

 one, the fore-end being less acute. A streak of pigment- 

 cells marks the middle of the iipper surface : darker cells mark 

 the origins of the nerves. These number from fifty to sixty 



168 



^i} n «fx?«i 



Ncr\'iins fltiros; from n sitft. or irrey nerve iu tlic 0;iir. ccvrr, 

 A, llbre resolving itself iiitu tll:iritice. li, a llln-e duubied on itseif, 

 sliowinff tlie flattened cbanicter. C, Two fibres lying in juxtaposi- 

 tion, a, a, o, nuclei, c, a nuclear fibre {Kerii/aser). d, a Ubrilla. 



pairs, and appear to come off as simple chords, fig. 170. They 

 perforate the memljranous neural canal, and accompany the inter- 

 muscular septa, dividing into two principal branches — one to the 

 neural or dorsal, the other to the hajmal or ventral, muscular 



109 



Diagr.am of Anatomy of tbc Lancelot, Dnrnchiostoma 



se^fments. The first pair of nerves, fig. 170, h, which Professor 

 Goodsir' thinks might correspond to the 'trifacial,' passes to the 

 membranous parts above the mouth : it may be the homologue of 

 that which, when a part of such membrane becomes specialised 

 as an olfactory sac, becomes the olfactory nerve, as, e. g., iu the 



