reissner's fiber in teleosts 223 



of the spinal cord throughout its whole length.^ Some of the 

 stages in this process are shown in figures 6 to 9. In this par- 

 ticular case, however, it is difficult to say whether these cells are 

 producing or regenerating the fiber. The fiber is fully formed in 

 trout embryos which are about half an inch in length (approxi- 

 mately fifty days old). 



Until recently very little work has been done on the regenera- 

 tion of Reissner's fiber. In this connection Sargent, first, and 

 afterwards Nicholls, showed that when the fiber is experi- 

 mentally severed it retracts toward the place of its attachment. 

 Nicholls has also stated that the fiber regenerates after such 

 severance, and also that in its recoil it takes on a spiral form, 

 as was also held by Sargent. He believes that the fiber is a living 

 protoplasmic structure, having inherent ability to uncoil, to re- 

 sume approximately its original form, and to establish new con- 

 nections with the 'terminal plug.' My own observations on this 

 subject are, as yet, very few, but I can confirm the statement 

 that the fiber recoils spirally when it is experimentally severed. 

 I have a few sections, too, which may be taken to suggest a 

 subsequent straightening of the fiber, although they do not 

 prove it, nor do they suggest the method by which the straight- 

 ening is effected. This process may be purely mechanical and 

 not the result of any inherent vitality, or it may even be brought 

 about chiefly through the agency of cells which enter the lumen 

 of the cord from the wall and seem to fuse completely with the 

 fiber (figs. 7 to 9). 



There has been. much controversy concerning the function of 

 Reissner's fiber. Sargent's theory that it is a nerve-fiber tract 



' In embryos which are about twenty-five days old there is a considerable 

 migration of cells from many portions of the brain wall into the lumen In 

 addition to the cells which form Reissner's fiber, there are blood corpuscles and 

 peculiar cells, particularly from the regions near the velum transversum, which, 

 entering the cavity of the brain, seem to disintegrate into a substance indis- 

 tinguishable microscopically from the coagulated cerebrospinal fluid. These 

 cells probably form an important constituent of the complex brain fluid. This 

 migration decreases with further development, but it has not been determined 

 yet whether it is repeated at later stages. 



