272 FISH. [Vol. X. 



in the Axolotl {Siredon). Burckhardt (2) states that these 

 fibers remain only in the larvae of Amphibia. They have been 

 investigated in the Teleosts by Mayser, and in the Petromyzon 

 by Ahlborn, who distinguishes them from Miillerian fibers by 

 their greater size. They have also been found later by Burck- 

 hardt (3) in the Protopterus, where they are of large size and 

 exhibit a fibrous structure, which in horizontal sections is seen 

 to be due to the entrance of fibers from without. 



Sanders (47) also finds them well developed in the Ceratodus, 

 but calls them multiaxial fibers. He investigates them at 

 some length and the following is quoted from him : 



" The peculiarity of their structure consists in the fact that 

 from forty to fifty axis cylinders are contained in a single medul- 

 lary sheath which is common to all of them. This sheath, not- 

 withstanding its immense extent and thickness, has the appear- 

 ance and structure of the medullary sheath of ordinary nerve 

 fibers. The axis cylinders also resemble those found in other 

 parts of the cord. 



" Fulliquet, who appears to have been the first to investigate 

 microscopically the brain of Protopterus, describes two large 

 fibers occupying a corresponding position in the spinal cord of 

 that animal ; these he terms Mauthner's fibers, but from his 

 plates and the terms in which he mentions them, I should 

 imagine that he refers to Miillerian fibers. The distinction is 

 essential, for while the latter are unprovided with medullary 

 sheaths, the former have them unusually well developed. 



" The multiaxial fibers commence at the posterior end of the 

 spinal cord, and are first met with opposite the hinder end of 

 the abdomen ; here they consist of a very few axis-cylinders 

 enclosed in a comparatively small medullary sheath ; the axis- 

 cylinders gradually increase in numbers as the fibers proceed 

 forward, but the increase is not uniform ; at about the middle 

 of their course they become much smaller, and after a few 

 sections again enlarge to the original diameter. But this 

 diminution does not occur at the corresponding point on the 

 opposite side ; but the fiber of one side first suffers a diminution, 

 then that of the other side. 



" Occasionally axis-cylinders may be seen escaping singly or 



