SARGENT: THE OPTIC KEFLEX APPARATUS OF VEUTEBUATES. 163 



it in selachians, but noticed a granular coagulum in the canal, which he 

 believed corresponded to it. It was not again noticed till Studnicka 

 ('99) found it in Alopias, Scyllium, Acanthias (young and adult), and 

 Chiraaera ; in the latter he observed a parallel splitting of the fibre. 



The cells from which the components of the fibre arise were dis- 

 covered in selachians by Rohon ('77, p. 38), and described by hiui as 

 lying for the most part on either side of the median plane. He found 

 the cells to be multipolar, each with a large eccentric nucleus and nucle- 

 olus. " Tiber die nahern Beziehungen derselben zu den sie umgebende 

 Elemente vermag ich nichts Bestimmtes anzugeben. Wohl verlaufen von 

 einer nicht unbetrachtliclien Anzahl Faserbiindel, zu deuen sich auch 

 die Fortsatze von dem die Dachkerne nach aussen umsaumenden Cylin- 

 derepithel zugesellen, aber sie alle zieheu immer iiber, unter und zwischen 

 den einzelnen Zellen hinweg." 



Sanders ('86, pp. 749-751) was the next investigator to notice these 

 cells. He described their occurrence and distribution with some mi- 

 nuteness in Scyllium, Rhina, Acanthias, and Raja, but otherwise added 

 little to our knowledge of them, and fell into error in describing them 

 as unipolar. " They generally give off one process only, or very rarely 

 two. Rohon imagined that he saw several processes cut off close to 

 many of these cells, and therefore came to the conclusion that they 

 were multipolar cells, but such is not the case." Sanders traced the 

 single process to the fibre-tracts of the middle layer of the tectum, 

 and says of these cells, " They may be probably looked upon as cor- 

 responding to the large cells in the nerve-cell layer of the I'etina," and 

 as having some function connected with the movements of the eye. 



Haller ('98, p. 513) describes the cells of the 'Dachkern'in Scyl- 

 lium as lying close to the ventricle and reaching back to the cerebellum. 

 Though he shows the cells in a number of his figures, he adds nothing 

 to our previous knowledge. In a later paper, Haller (:00, p. 277) 

 arrives at the conclusion that the ' Dachkern ' replaces (' ersetz ') the 

 ' nucleus corticalis ' of teleosts, which is absent in selachians, and like 

 it gives rise to two ' Associationsbahnen,' which run to the ventral part 

 of the brain and ai*e lost. 



Edinger (:01, p. 6G8) noticed this roof-nucleus in Scyllium, naming 

 it the ' nucleus magnocellularis tecti,' and described the tract of fibres 

 arising from it and running to the cerebellum. He fell into the error 

 of Rabl-Ruckhard and Osborn, believing it to be connected with the 

 roots of the trigeminus nerve. 



Finally, Houser (:0l), in his recent monograph, has fully confirmed 



