SAEGENT : THE OPTIC REFLEX APPARATUS OF VERTEBRATES. 199 



of the roof (Plate 7, Fig. 47 ; Plate 8, Figs. 55, 56, 58, /as. lieis.), even 

 before the posterior comnnssure has developed. In the later develop- 

 ment the fibres of the posterior commissure, in making their way across 

 from the opposite sides, pass posterior to these axons. Such a pre- 

 posterior commissural tract, the tractus toro-fibrae Reissneris anterior 

 (Figs. /, /, K; Plate 7, Figs. 48, 51 ; Plate 8, Figs. 56, 57, 59 ; 

 Plate 10, Fig. 69 ; frf. for. fhr. Beis. a.) is found only in teleosts, 

 though its beginning is foreshadowed in Polypterus (Plate 6, Fig. 41). 



C. The Optic Reflex Apparatus. 



SiLURiDAE (34).-^ In the Siluridae I have studied this apparatus in 

 a series of larvae of Ameiurus nebulosus extending from the time of 

 hatching to larvae 3 centimetres in length. The brain in the Siluridae 

 is of relatively simple morphological structure, much the simplest of all 

 teleosts. Its structure is transitional between the primitive type of the 

 ganoidean brain and the aberrant complexity of the teleostean brain. 

 The optic lobes are but little more developed than in the ganoids. 

 The relations in the anterior part of the optic lobe are much the same as 

 in Amia (Plate 7, Fig. 47). 



The torus longitudinalis is late in developing, and even in the adult 

 is of small size and primitive structure, resembling the condition in 

 ganoids. Each lobe appears as the median portion of the optic tectum 

 slightly constricted off from the lateral portion by a shallow groove. 

 Anteriorly the lobes of the torus converge and fuse in the median plane. 

 Posteriorly they diverge till they are widely separated, being connected 

 by only the thin median roof of the ventricle (Plate 9, Fig. 65). 



The optic reflex apparatus is already well established at the time of 

 hatching and reaches full development soon after. In larvae 1 to 2 

 centimetres in length Reissner's fibre has been traced through the whole 

 length of the central canal, and has a diameter of from 5 to 8 micra. 

 Reissner's fibre runs directly cephalad through the optic ventricle to its 

 anterior portion just behind the posterior commissure, where it breaks 

 up into many branches. Numerous fine fibrils emerge from the anterior 

 portion of the roof of the ventricle near the median plane and extend 

 towards the fibre (Plate 7, Fig. 47, fas. Beis.). 



The optic reflex cells arise for the most part in the anterior end of 

 the roof, and lie in the mesal portion of each half of the tectum, that 



1 The numbers given with the name of the family are those assigned to that 

 family by Jordan and Evemiimn ('96-00), and will serve roughly to indicate the 

 relationships of the several families. 



