204 BULLETIN : MUSEUM OF COMPAIIATIVE ZOOLOGY. 



coele where the axons enter is reduced to a mere canal, owing to the 

 union of the halves of the tectum below the torus (Figs. E, F). Through 

 this the finer trunks of Reissner's fibre make their way into the anterior 

 recess of the mesocoele. 



The cells lying in the anterior, dorsal, and lateral portions of the torus 

 send their axons into the ventricle by the shortest path. In the growth 

 of the tectum eephalad these cells have come to lie far cephalad of the 

 posterior commissure, so that the shortest path to the ventricle lies 

 anterior to the posterior commissure (Plate 7, Fig. 51 ; Fig. 7i). The 

 axons taking this course are aggregated into two lateral bundles, the 

 tractus toro-fibrae Keissneris anterior. These tracts, converging toward 

 the median plane, enter the ventricle as compact bundles, in which the 

 constituent axons can be recognized only in very early stages of develop- 

 ment. In larval stages this tract within the ventricle consists of a num- 

 ber of divisions, which in the adult become consolidated, so that the 

 greater portion of Reissner's fibre emerges into the ventricle at one 

 point (Fig. 51). In Salmo and Salvelinus the greater number of the 

 constituent axons of Reissner's fibre enter the ventricle by this path (trt. 

 tor. fbr. lieis. a.). 



Ventral and caudad to the posterior commissure these two sets of 

 constituents meet and coalesce. In larval stages the anterior portion of 

 Reissner's fibre consists of many divisions, and one frequently finds in 

 sections lying within the mesocoele loosely tangled masses of these fibre- 

 like divisions which have been displaced in cutting. This diffuse 

 branching of Reissner's fibre is much reduced in older fry, and in the 

 adult only a few small divisions may be found before the main trunk of 

 the fibre enters the brain tissue anterior to the posterior commissure. 



In the adult, Reissner's fibre in its course through the mesocoele is 

 surrounded by a loose membranous sheath, which extends almost to the 

 metacoele. Tliis is formed from the membrane lining the ventricle by 

 its deflection outward upon the fibre. 



In its course through the brain ventricles and central canal Reissner's 

 fibre appears as a sharply defined rod of conspicuous size. In trout fry 

 2 cm. long, the rod has a diameter of 0.3 micra. In the adult trout the 

 diameter is 3 micra. 



There is no well-marked ventriculus terminalis in the Salmonidae 

 but at the posterior end of the canalis centralis, where the spinal cord 

 is bent upward into the hetei-ocercal tail, the posterior canal-cells can 

 with some difficulty be distinguished, attached by their axons to the 

 fibre as grapes to their stem. 



