196 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



four investigators, Stieda, Sanders, Mayser, and Studnicka. Stieda ('68, 

 p. 11) described it as occurring in the central canal of all teleosts 

 that he had examined (Gadus, Cyprinus, perch, shad, eel, and pike). 

 In his Taf. 1, Fig. 9, he has shown it within the canal in a transverse 

 section of the cord of the pike, and in Fig. 12 in a longitudinal section 

 of the cord of Gadus lota. Sanders ('78, p. 742) desci'ibed a rod within 

 the canal, which he found occasionally in sections of the cord of the 

 mullet (Mugil). 



Mayser ('82, p. 295) found Eeissner's fibre in the fourth ventricle 

 in cyprinoids, and traced it forward to the region of the trigemiual 

 nerve. In his Figuren 20 und 21 he has shown it in cross-sections of 

 the cord lying within the central canal. Studnicka (99, p. 3) merely 

 mentions its occurrence in Anguilla. 



Stieda and Sanders believed tlie rod wliich they saw within the canal 

 to be an artifact formed by the action of the fixing fluids on the contents 

 of the canal. Mayser and Studnicka looked upon lleissuer's fibre as a 

 preformed structure existing during the life of the animal. 



Bellonci ('8l), Fusari ('87), and C. L, Herrick ('9l) observed the fibre- 

 tracts from the torus to the tectum ; and Sala ('95), P. Cajal ('99), and 

 Catois (:0l) studied the cells of the torus, without, however, suspecting 

 their true relations or function. The only other references to any 

 portion of the optic reflex apparatus in the literature of the teleost 

 brain are in my previous papers (Sargent, :00, :00% :01, :01*). 



B. MORPHOLOGT OF THE MESENCEPHALON. 



1. The Median Zone. Burckhardt ('94) pointed out that the 

 median longitudinal zone of the brain retained to a greater degree than 

 any other portion, the primitive characteristics of the neural tube, re- 

 maining fairly constant, both in ontogeny and phylogeny, throughout the 

 vertebrate series. The median zone is essentially ependymal, while the 

 bordering lateral zones are highly nervous. 



That portion of the median roof lying anterior to the posterior com- 

 missure, and between it and the base of tlie epiphysis, the pars inter- 

 calatns, designated by Burckhardt as the " Schaltsttlck," is in teleosts 

 in an especially primitive condition (Fig. /). It is of considerable 

 lateral breadth, and is for the most part made up of the thickened epen- 

 dyma of this region (Plate 7, Figs. 47-52, eras, e'end.), consisting of 

 elongated, cylindrical, ependymal cells inclined obliquely ventrad and 

 caudad. These constitute an inverted trougli, or groove. This ependy- 

 mal groove (Figs. E^ F) is, however, not so prominent as in the 



