39 
ent, opt. 
lob. f. tor. lg. | tet. opt. vlv. cbl, cdl, 
/ 
/ 
coll. 
ity Ie 
Fig. 1. Sagittal section of brain of Cynoscion regale (squeteague); outlines with 
the aid of camera lucida. ° 
ebl. cerebellum. coll, colliculus. fbr. R. REISSNER’s fibre. Job. f. frontal lobe. 
lob. vag. vagus lobe. tct, opt. tectum opticum. tor. dg. torus longitudinalis. vv, cdl. val- 
vula cerebelli, wnt. opt. ventricle of optic lobes. 
sections of REISSNER’s fibre in Cynoscion, when double stained with 
EnrLicH’s hematoxylin and Congo red, show a very definite internal 
structure. The medullary portion takes the Congo red evenly, and by 
careful focussing under a 1/,, oil immersion presents a very finely 
granular appearance. Surrounding this is a hyaline layer, which takes 
the stain very lightly, if at all. Outside this, on the periphery, is a 
sheath taking a deep hematoxylin stain (Fig. 11). The existence of this 
darkstaining sheath is often apparent in cross sections stained by other 
methods, and also in longitudinal sections. Occasionally in cross section 
there may be seen coming from the fibre very fine filamentous pro- 
cesses, which run toward the periphery of the canal. These have 
a diameter of probably less than 0,1 «. I shall have occasion to de- 
scribe these further on. 
As already stated, Reıssner’s fibre extends through the whole 
length of the canalis centralis of the cord and continues cephalad 
through the 4th and 3d ventricles to the anterior end of the optic lobes. 
Posteriorly the fibre can be traced with little diminution in size, until 
near the extreme posterior end, as far, indeed, as the lumen of the 
canal can be distinguished. I have studied this portion of the fibre 
particularly in larval Amia and Lepidosteus from 1 to 2 cm in length. 
In an Amia 17 mm long the lumen of the canalis centralis in the 
middle region of the body has a diameter of 16 u, the fibre that of 
