144 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



(5) The posterior canal cells, in the ventricnlus terrainalis and pos- 

 terior portion of the central canal. The number of these varies in 

 different species from 6 to 40 (Plate 3, Fig. 22 ; Plate 4, Figs. 27-31). 



(6) The fibre of Reissner, with its anterior divisions and posterior 

 branches, (For photomicrographs of the fibre in situ, see Sargent, :00.) 

 The fibre extends from the anterior end of the mesocoele caudad 

 through the ventricles and the central canal to the ventriculus terrai- 

 nalis, usually lying near the floor of the ventricle or canal. Normally 

 the course of the fibre through the ventricles and canal is perfectly 

 straight (Sargent, :00, Figs. 2 and 3). Occasionally in sagittal sections 

 it has an undulating course (Sargent, :00, Fig. 4), and sometimes it may 

 be found coiled upon itself in a snarl or tangle (Plate 1, Fig. 8), this 

 latter condition being due to the recoil and contraction of the fibre 

 when the cord was cut in removing the brain. Studnicka (:00) has 

 described such a tangle formed by Reissner's fibre within the ventri- 

 culus terminalis of Petromyzon, and believes this to be the normal 

 condition. 



a. Appearance in the Fresh Condition. Not only has the fibre of 

 Eeissner been studied in situ, but I have been able to remove it from 

 the canal or ventricle, and study it thus isolated with the aid of 

 methylen-blue staining. Some of the larger selachians (Charchariaa 

 and Carcharhinus), in which the fibre has a diameter of from 15 to 25 

 micra, offer the most favorable subjects for such treatment. The fibre 

 is almost perfectly transparent, but may be readily seen in a strong 

 light because of its high refractivity. It has a considerable degree of 

 tenacity and is somewhat elastic, so that it can be drawn across a 

 glass slide. A number of permanent preparations were made in this 

 way. 



h. Appearance in Sections. After fixing, the fibre is usually brittle, 

 and in cutting may break at some distance from the plane of the section. 

 Under such conditions it breaks sharply at right angles to its length 

 (Sargent, :00, Fig. 8). Occasionally in longitudinal sections of the cord 

 cutting the fibre obliquely, the cut ends of the fibre are bent in the 

 direction taken by the knife, much as a soft wire might be. Frequently 

 in longitudinal sections the fibre is displaced in the cutting, so that it 

 may lie close to the wall of the canal. Sometimes the deeply stained 

 marginal portion of the wall of the canal may, in cutting, be torn off 

 and displaced so as to give somewhat the appearance of a fibre in the 

 canal. Such conditions may possibly account for Studnicka's concep- 

 tion of the nature of Reissner's fibre, he believing it to be a secretion 



