reissner's fiber in teleosts 2!21 



in the region of the sinus terminalis. It is a very fine (0.4 n in 

 diameter) taut thread, lying in the lumen and, usually, somewhat 

 nearer the ventral than the dorsal wall of the central canal. 

 The exact relation of the fiber to the cells which constitute the 

 walls of the spinal cord is often obscured by what appears to be 

 a coagulum. However, by a partially successful process, aimed 

 at removing the fluid of the central canal before fixation, and by 

 then holding the fishes in a vertical position during fixation, it 

 was often possible to obtain longitudinal sections in the median 

 plane of the cord which were fairly satisfactory in this respect. 



Many of the ependymal cells, particularly in the roof of the 

 cord, possess a structure which is somewhat similar to that of 

 the cells in the ' sub-commissural organ.' Some of them send a 

 fibril into the lumen of the cord. This fibril varies in length 

 in different cells. In some cases it does not extend to Reissner's 

 fiber, while in others it passes beyond. Sometimes, also, there 

 seems, at first sight, to be a connection between the two. Ac- 

 cording to my observations, however, none of these fibrils has a 

 structural union with Reissner's fiber, which seems, in the stages 

 I have studied, to be free from the walls of the cord throughout 

 its whole length (figs. 6 and 7). 



The posterior end of the spinal cord, the region of the sinus 

 terminalis, is not closed by the cells of the neural wall, and 

 Reissner's fiber, often somewhat enlarged, passes through the 

 opening thus left to end in a 'terminal plug,' slightly posterior to 

 the end of the neural tube. This plug is in close connection, 

 probably in continuity, with the connective tissue which sur- 

 rounds the spinal cord in this region (figs. 8 to 10). It may be 

 that the fiber here also has some histological connection with the 

 membrana externa of the neural tube. 



In its staining properties the fiber and its related structures 

 resemble most closely neuroglia and connective tissues. Although 

 a wholly satisfactory selective stain for the fiber is yet to be 

 found, it may be said that the fiber, as well as the fibrils from 

 the ependymal cells and also the brain membranes, all stain 

 characteristically and strongly in dyes which are supposedly 

 specific for elastin (e.g., in resorcin-f uchsin) . In rather unsatis- 



