SARGENT: THE OPTIC REFLEX APPARATUS OF VERTEBRATES. 165 



recalling the bilateral arrangement of the ependymal grooves in that 

 genus. This peculiar recessus is lined throughout by the characteristic 

 thickened ependyma of the pars intercalatus (' Schaltstiick '). Posteri- 

 orly this ependyma ends in a prominence, less cone-like than in the larval 

 condition. This ependymal thickening extends cephalad of the posterior 

 commissure, having in cross-section the appearance of a semicircular or 

 horseshoe-shaped arch at the summit of the narrow slit-like passage 

 connecting the mesocoele and diacoele (Plate 2, Fig. 14). It forms 

 the greater thickness of the pars intercalatus, which in selachians is 

 relatively much shorter than in ganoids and teleosts, having been 

 encroached upon by the posterior commissure (Fig. 16). The ependymal 

 thickening continues through the diencephalic roof into the recessus sub- 

 pinealis, and in contact with the ganglia habenulae merges into the usual 

 ependyma which lines the ventricles. The ventricular surface of this 

 thickening is thrown into transverse folds (Fig. 16), or penetrated by 

 finger-like recesses opening posteriorly (Plate 2, Fig. 10). The ependyma 

 consists of columnar cells each with a conspicuous nucleus, which is 

 somewhat removed from the ventricular space. 



h. Cells of the ' Dachkeni.' The ' Dachkern ' cells lie in the median 

 portion of the mesencephalic roof, extending throughout its length 

 immediately above the ventricle (Plate 3, Fig. 15, nidi, id.) and separated 

 from it by only a thin layer of ependymal and neuroglia cells (Plate 3, 

 Fig. 18). Frequently one of the large cells is in immediate proximity 

 to the ventricle or projects into it. Immediately dorsal to this nidulus 

 the dorsal decussation of the tectum passes from the stratum medullare 

 profundum of one side to that of the other (Plate 3, Figs. 15, 18, 19, 

 20, deed.). The cells are most numerous through the middle half of 

 the tectum (Fig. 15), but they extend forward in lesser number to the 

 border of the posterior commissure (Fig. 16), and backward through 

 the tectum quite to the cerebellum (Fig. 21). Anteriorly the cells are 

 most numerous on either side of the median plane, where they form 

 a two-layer nidulus connected by a lesser number of cells in the median 

 plane. Posteriorly this bilateral arrangement disappears, and the cells 

 form a single hand in the median plane. A few straggling isolated cells 

 may occur laterally in the tectum even at some distance from the main 

 nidulus. 



The number of cells in this roof nidulus is probably between four and 

 five hundred. In a young specimen of R. erinacea, five hundred and 

 seventy-two cells were counted. As portions of each cell occur in 

 several sections, the attempt was made to count only those whore the 



