172 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



extending dorsad, but not far enough to lie on its dorsal side as in 

 Raja (Fig. 16). The ventral portion of this figure is strictly median. 

 At its posterior end (Fig. 10, e'emZ.) the demarcation between this 

 specialized ependyraa and that which lines the rest of the mesocoele is 

 abrupt. At the anterior end there is a more gradual transition into 

 the vascular epithelium of the epiphysial stalk. 



(2) The Cells of the ' Dachkern.^ The cells of the ' Dachkern ' lie 

 in the raesal portion of the tectum on either side of, and in, the median 

 plane (Fig. 9), but are incompletely separable into two lateral niduli. 

 The cells lie close to the ventricle, immediately above the ependymal 

 lining of the roof. Near the median plane they form but a single layer 

 (Fig. 9), but more laterally two or three cells may be superposed 

 (Plate 3, Fig. 21). They extend from the extreme posterior part of the 

 tectum, in contact with the cerebellum, cephalad to within a short dis- 

 tance of the posterior commissure, but are more numerous and attain 

 greater size in the posterior two-thirds of the tectum. The cells seem 

 to have a tendency to migrate posteriorly. A few are always to be 

 found beneath the trochlearis commissure (Figs. 9, ll,r(), as has been 

 noted by Haller ('98). Their number is considerably less than in Raja, 

 probably not exceeding two hundred. Since they are less crowded, they 

 are more regular in form (Fig. 21). The spherical or ovoid form, modi- 

 fied by the multipolarity of the cell, predominates. The optic lobes of 

 Mustelus are larger than those of Raja, and the 'Dachkern' cells are 

 of proportionally greater size, attaining a maximum diameter of 200 

 micra. 



The nucleus is of sharper outline than in Raja, and almost uniformly 

 spherical. It is eccentric and occasionally, as in Raja, is pushed far out 

 from the centre of the cell so as to form a protrusion. It contains one 

 or more nucleoli, around which the chromatin forms a reticulum 

 (Fig. 21). The cytoplasm, as in Raja, is minutely granular, but near 

 the surface of the cell the Nissl's granules are more numerous and of 

 larger size. The neuroglia capsule surrounding the cell is not so 

 markedly developed as in Raja. 



Upon cursory examination the cells appear unipolar or bipolar 

 (Fig. 21, a, c). More careful study shows them to be multipolar, as in 

 Raja, though the number of fine dendrites arising from the body of the 

 cell is less than in Raja. Frequently the axon and cerebellar neurite 

 arise from the T-shaped division of a single process, but more commonly 

 they come off from the opposite ends of the cell (Plate 2, Fig. 9). The 

 typical cell is ovoid, its long axis lying parallel Avith the roof of the 



