178 bulletin: museum of comparative zoology. 



D. Summary fob Selachians. '' 



Judged by the relative size of its eleaients, the optic reflex apparatus 

 reaches perhaps its highest development in the selachians. Its cell 

 nidulus, known since the time of Rohon as the * Dachkern,' extends 

 on either side of the median plane through the length of the optic lobes, 

 close to the ventricle. 



The cells of the ' Dachkern ' are distinguishable at an early stage 

 of development, but do not reach a functional condition until tlie young 

 attains the free life, — a relatively much later stage of development than 

 that at whicii it becomes functional in ganoids and teleosts. The 

 habenular (olfactory) constituents of Keissner's fibre are developed and 

 probably functional before the optic elements arise. 



The ependymal groove, hitherto unnoticed in selachians, is highly 

 developed and forms a conspicuous structure, in Raja extending up 

 into the narrow recess above the posterior commissure. In Mustelus 

 it has a greater lateral extent, and is curved upon itself so as to form 

 a partially enclosed tube, horseshoe-shaped in transverse section. In 

 Raja it still preserves in part the bilateral arrangement so conspicuous 

 in cyclostomes. 



The cells of the optic reflex aj)paratus constitute the so-called ' Dach- 

 kern,' a nidulus of giant cells lying in the tectum opticum immediately 

 ventral to the dorsal decussation, on either side of the median plane and 

 extending caudad to the cerebellum. They are more numerous and of 

 relatively larger size in Raja than in Squalus or Mustelus. The cells are 

 variable in form. Each cell is surrounded by a pericellular capsule of 

 neuroglia and ependymal fibres. Fine nerve-fibres, probably from the 

 dorsal decussation, end in arborizations over the body of the cell. The 

 nucleus is always eccentric and opposite the largest process. The cyto- 

 plasm is finely granular, the Nissl's granules being most conspicuous 

 near the periphery of the cell. 



The cells are multipolar, and give rise ultimately to three principal 

 processes. The smallest passes with the fibres of the dorsal decussa- 

 tion into the stratum raedullare profundum, where they come in con- 

 tact with the endings of the retinal fibres of the optic nerve. A second, 

 coarser process, directed caudad, goes to form, on either side of the 

 median plane, the tractus tecto-cerebellaris, which breaks up in the 

 molecular layer of the cerebellum. A portion at least of these fibres 

 decussate on entering the cerebellum. The third process, directed 

 cephalad, forms with others of its kind the tractus tecto-fibrae Reiss- 



