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 



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

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

 showing the flattened character,. C, Two fibres lying in juxtaposi- 

 tion, a, a, a, nuclei, c, a nuclear fibre (JZernfaaer). d, a fibrilla. 



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 hamial or ventral, muscular 



1G9 



Diagram of Anatomy of the Lancelet, Branchiostoma 



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

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



xc. 



