206 



BULLETIX: MUSEUM OF COMPAKATIYE ZOOLOGY. 



optic lobes and tracts distinctly degenerate ; the tectum is only one-half 

 to one-t]iii"d as thick as in the normal brain. The torus longitudinalis 

 is in a state of arrested development, similar to that of other teleosts 

 before hatching, and scarcely exists as an independent structure (Plate 

 9, Fig. 64). It is marked off from the tectum by a shallow fissure, and 

 contains few or no nerve-cells, consisting almost wholly of ependyma. 

 The dorsal decussation of the tectum passing through it occupies the 

 greater part of its thickness (Fig. G, dec. d.). 



Keissner's fibre has been found only within the central canal, where 

 it is an exceedingly delicate filament, which, in young specimens 8 



Fig. G. 



Fig. H. 



Figure G. Amblyopsis spelaeus, adult. Transverse section of the tectum opticum 



and torus longitudinalis. 

 Figure H. Fundulus heteroclitus, adult. Transverse section of the tectum and 



torus. For meaning of abbreviations, see Explanation of Plates, p. 257. 



to 10 mm. long, has a diameter of 0.1 micron (measured with a Zeiss 

 micrometer ocular and apochromatic objective). In larger specimens, 

 30 mm. long, the diameter was somewhat greater. The constituents of 

 the fibre in this case probably come in large part if not wholly from the 

 ganglia habenulae, though I have been unable to trace a direct con- 

 nection in this species. As the eyes in Amblyopsis are totally useless, 

 the much-reduced Keissner's fibre is probably, as in the blind Myxine, 

 exclusively a reflex fibre-tract between the olfactory centres of the 

 habenulae and the musculature. "With this degeneration of the optic 

 reflex apparatus there is apparently an increased development of the 

 reflex apparatus of which Mauthner's fibres are the conducting tracts. 



