42 
torus longitudinalis, which projects ventrad and caudad from the an- 
terior wall of this ventricle (Fig. 1). Passing along the median fissure 
of the torus for one-half to two-thirds its length and close to its surface 
(Figs. 8, 9), the fibre passes beneath the membrane which covers the 
torus and enters the brain substance (Fig. 10). In Cynoscion and 
Salvelinus the fibre after passing beneath the membrane may be fol- 
lowed for 100 u or more before it breaks up. In the anterior portion 
of the ventricle of the optic lobes, the fibre is often surrounded by a 
loose membranous sheath not found elsewhere. This is probably a 
prolongation on to the fibre of the membrane enveloping the torus. 
Ventral to the torus ReISsNER’s fibre may divide once or twice before 
entering the torus (Fig. 8). In the squeteague (Cynoscion) such a branch- 
ing has been observed in studying both sagittal and transverse sections. 
In this species the branches are of unequal size. In a series of trans- 
verse sections a fibre 6 « in diameter was seen to divide, the branches 
being respectively 4 « and 2 w in diameter. Both branches entered 
the torus, the smaller at a point a little anterior to the larger. In 
another series of the same species there was evidence of a third and 
smaller branch. This division of the fibre before entering the torus 
has also been observed in Pomatomus and Pseudopleuronectes. 
Reıssner’s fibre, then, after entering the torus continues cephalad 
for some distance just within the membrane covering the torus (Fig. 6), 
and is then deflected into the deeper parts of the body. Just what becomes 
of it from this point on cannot be definitely stated at present. The 
evidence, however, so far as it goes, indicates that it branches repeat- 
edly, and that the fine branches thus arising are intimately associated 
with the gelatinous tracts of the torus, which, according to Herrick (92), 
run to the ectal side of the tectum opticum. 
Cambridge, Mass., June 1899. 
Postscript. — After the manuscript of this paper was sent to the 
editor there was an unexpected delay in procuring the photographs 
of my sections to illustrate it. In the meantime SrupniéKa has 
published an article on this subject — “Der Retssner’sche Faden aus 
dem Centralkanal des Riickenmarkes und sein Verhalten im Ventri- 
culus terminalis” in Sitzungsber. béhm. Gesellsch. Wiss., math.-naturw. 
Cl., July 1899. 
His conclusions as to the nature of the fibre are drawn princip- 
ally from the study of it in Petromyzon, though he has identified it 
in other animals. His view that the fibre is the result of secretion 
