ITS ONTOGENY, MORPHOLOGY, PHYLOGENY, AND FUNCTION. 405 



In transverse section each lobe is triangular or pear-shaped, the narrow end being 

 at the place of connection with the tectum. 



In the Salmonidse the optic lobes are of great size. The tectum opticum has 

 a relatively enormous development both in area and thickness. It extends cephalad 

 of the posterior commissure so as to overlie partly the diencephalon. The mesocoele 

 is prolonged cephalad into a recessus (Fig. 20, rec. ms'ccel.) above the posterior com- 

 missure. The torus attains its greatest development anteriorly above the posterior 

 commissure, but extends caudad nearly to the posterior limit of the mesencephalon. 

 The tectum has so great a thickness thatjthe torus does not^reach to the level of the 

 roof of the mesocoele, but lies entirely between the two halves of the tectum (Figs. 

 2, 3). Through the anterior half of the mesencephalon the mesal edges of the two 

 halves of the tectum meet below the torus so that the torus is embedded between them 

 (Fig. 2). As the result of lateral pressure the torus has become laterally flattened so 

 as to appear in transverse section dorsoventrally elongated and the median ventral 

 longitudinal fissure dividing the torus into two lateral halves is almost obliterated. 

 Auerbach ('88) failed to recognize the torus in this form and asserted that it was absent. 

 In his figures of young trout he has obviously shown the torus, describing it as "in 

 das Tectum eingebettete Zellstrang." That which he designates as the "Anlage 

 des Torus longitudinalis " is the inner and ventral edge of the tectum where it meets 

 the mesal edge of the tectum of the opposite side below the torus (Figs. 1, 2, 3, a.). 



The Amblyopsidse is a group of small fishes, mostly cave-inhabiting, having 

 degenerate eyes, some of which are totally blind (compare Eigenmann, '99). The 

 degeneration of the eye is accompanied by profound changes in the central nervous 

 system, more especially the atrophy of the structures of the dorsal portion of the 

 mesencephalon. In Chologaster papilliferus, which has degenerate eyes though still 

 functional, the optic lobes are much smaller than in the closely related Fundulus, 

 which has normal eyes. The tectum is thin and the torus lobes are but little 

 developed. In three other genera which are totally blind, having only vestigeal 

 eyes, the torus is scarcely developed as an independent structure. 



The optic centres of the central nervous system of Amblyopsis have recently 

 been studied by Ramsay (:01), who finds the optic lobes and tracts distinctly degen- 

 erate. The tectum is only one-half to one-third as thick as in the normal brain. 

 The torus longitudinalis is in a state of arrested development, resembling the condi- 

 tion in other teleosts before hatching, and reminiscent of its form in the Siluridse. 

 It equals the thin tectum in thickness, but does not extend below it as in most tele- 

 osts. It is marked off from the tectum by a shallow fissure, about equal in depth to its 

 median ventral fissure. The dorsal decussation of the tectum passing through it 



