174 bulletin: museum of comparative zoology. 



ventricle ventral to the posterior commissure, passing through the 

 ependymal thickening, which probably serves as a supporting structure 

 and • anchorage ' for the fibre. In the anterior portion of the ependy- 

 mal groove (Fig. 10, sul. e^end.) one or two fine trunks have been traced 

 from the base of the ganglia habenulae din. hah.) caudad in the gi'oove 

 to where they unite witli tlie anterior divisions of Eeissner's fibre. 



Just caudad of the posterior commissure all these constituent trunks 

 unite to form Eeissner's fibre (Fig. 10, fhr. Rek.), wliich from this 

 point is easily traced caudad through the ventricles and central canal. 

 Its diameter in the adult Squalus is 6 to 8 micra, but in the dusky 

 shark, Carcharhinus obscurus, 8 ft. long, it attains a diameter of 25 

 micra. In one series of sections from the posterior portion of a Squalus 

 embryo, 5 cm. long, I have found Reissner's fibre coiled within the 

 posterior end of the central canal, in much the same condition as that 

 described for Petromyzon (Plate 1, Fig. 8). 



In its course through the canal, particularly in the caudal portion, 

 Eeissner's fibre gives off fine brandies. These are generally directed 

 obliquely caudad and in traversing the canal frequently undergo divi- 

 sion ; the several resulting fibrils enter the cord between the ependymal 

 cells of the latero-ventral walls of the canal (Plate 9, Fig. 67). 



c. Posterior Canal-Cells. At its posterior extremity the central canal 

 abruptly dilates into a large oval, or nearly spherical, terminal ven- 

 tricle (Plate 3, Fig. 22). In embryos up to 5 cm. in length there 

 remains an out-pocketing at the caudo-ventral extremity of the ventricle, 

 the remnant of the neurenteric canal (Fig. 22, can. vJenr.). Within the 

 ventricle, for the most part in its ventral half, there are from six to 

 eight posterior canal-cells, and within the adjacent portion of the canal 

 there are from ten to twenty additional cells of much smaller size 

 (Plate 9, Fig. 67 ; Plate 3, Fig. 22). The cyto^jlasm is diffuse, taking 

 stains lightly. The large oval nucleus is sharply defined, and usually 

 has a single nucleolus, around which the chromatin is arranged in a 

 reticulum. The cells are multipolar, their several dendrites passing 

 into the wall of the canal or of the terminal ventricle. The axons, in 

 passing cephalad through the terminal ventricle and canal, unite with 

 one another. In the posterior portion of the canal they eventually fuse 

 with Eeissner's fibre. Figure 22 shows the character and relations of 

 these cells. It is a composite drawing made from four successive 

 sections. Each section was drawn by camera in outline on tracing- 

 paper^ and the outlines superposed to get the relative positions of the 

 cells. 



