SARGENT : THE OPTIC REFLEX APPARATUS OF VERTEBRATES. 133 



due to the fact that in the study of the spinal cord transverse sections, 

 in which this rod is very inconspicuous, have been so hirgely relied on. 

 However, recent investigators who have studied the central nervous sys- 

 tem by means of longitudinal sections have also failed to note this struc- 

 ture, probably for the reasons that (1) many staining methods fail to 

 bring it out clearly, and (2), being a structure of great delicacy and one 

 of the first to bi'eak up after death, it shows clearly only in perfectly 

 fresh material carefully fixed. 



The fibre in question was discovered by Reissner ('60), who described 

 it as a highly refractive cylindrical rod, 1.5 micra in diameter, lying 

 within the lumen of the central canal of Petromyzon. " Da dieser 

 Strang, wenn ich ihn iiberhaupt zu Gesicht bekam, stets von derselben 

 Gestalt war, und nicht einmal Formverschiedenheiteu darbot, welche 

 die Axencylinder an Chromsiiurepniparaten so haufig zeigen, kann ich 

 nicht annehmen, dass er gleichbedeutend sei niit den iinregelmassigen 

 Massen, welche den Centralcanal bisweilen vollstiindig oder zum Theil 

 erfiillt und im Riickenniark andei'e Thiere oder des Menschen von 

 mehreren Forschern erwahnt vvorden sind." He denied that it could 

 have been formed from thrown off epithelial cells or blood corpuscles, 

 as similar contents of the canal had been accounted for by Stilling ('59). 

 If, he said, it was formed from the coagulation of the albumen of the 

 cerebro-spinal fluid, as Bidder und Kupffer ('57) had supposed, then 

 one must acknowledge that the chemical composition of the cerebro- 

 spinal fluid was the same, or nearly the same, as the substance which 

 forms the axis-cylinders. In regard to its origin and ending, Reissner 

 could find nothing, and he knew of its occurrence in the cord of Petromy- 

 zon only. 



Kutschin ('63) confirmed for Petromyzon Reissner 's discovery, nam- 

 ing the structure Reissner's fibre, but failed to add anything to previous 

 knowledge of it. 



Stieda ('68) found Reissner's fibre present in all teleosts examined by 

 hiiu. He describes it as a cylindrical rod of homogeneous structure, 

 having in general a diameter of 3.8 micra. Stieda considered it to be 

 an artifact produced by the chromic acid used in fixation. 



Later Stieda ('73) observed the fibre in selachians also. He says 

 ('73, p. 438) : " Auch hier fand ich sowohl auf Quer- als auf Liingsschnit- 

 ten mitunter im Lumen jenen rathselhaften Strang, welcher einem 

 Achsencylinder im Aussehen gleicht. Ich habe den Strang fast bei 

 alien untersuchten Wirbelthieren gesehen, und habe mich an einem 

 andei'n Orte dahin ausgesprochen, dass derselbe ein Kunstproduct, d. h. 



