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 upper surface : darker cells mark 

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



168 





B 



a 



Xvrvous fibres from a soft or grey nerve in the Calf. ccvu. 

 A, fibre resolving itself into fibrilhe. B, a fibre doubled on itself, 

 showing the tlnttmed character. C, Two (Hires lying in juxtaposi- 

 tion, a, a, a, nuclei, c, a nuclear fibre (Kernfaser). d, a llbrilla. 



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

 perforate the membranous neural canal, and accompany the inter- 

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

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



169 



"f 



Diagram of Anatomy of the Lancelet, Brcmchiostoma 



segments. The first pair of nerves, fig. 170, b, which Professor 

 Goodsir * thinks might correspond to the f trifacinl,' 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., in the 



