REISSNER S FIBER IN TELEOSTS 



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From the deep end of each of these cells a single, nearly 

 homogeneous, highly refractive process (fbrl.) extends to the 

 membrana limitans externa of the brain, with which it fuses 

 (figs. 1 to 5 and B). So far as I have been able to determine, 

 none of these cells give rise to processes which turn into the 

 nerve tracts of the posterior commissure. The cells usually are 

 arranged somewhat as in a typical columnar epithelium (fig. 1), 

 but in the later stages their axes become more and more oblique 

 to the inner surface of the brain wall whence the fibrillae emerge 

 (fig. 3). The connections of the processes, however, are not 

 altered. 



Fig. A General view of sagittal section of brain, to show the region of the 

 posterior commissure and also a probable migration, from the roof of the brain, 

 of cells concerned in the formation of Reissner's fiber. The cells believed to be 

 migrating have been projected on the plane of the section from another individual 

 than that which furnished the outline. 50 days. 



By making thick (30 fx to 40 m) frozen sections of the brains of 

 young trout, it was possible under a lens to isolate the region of 

 the posterior commissure and the epithelial wall covering it. 

 Such isolated regions, when properly treated with dissociation 

 reagents, gave good 'isolation preparations' of the epithelial 

 cells now under consideration. These cells are larger than the 

 typical nerve cells. All of them seem to possess a characteristic 

 structure. The deep end of the cell body is produced into the 

 process already mentioned, which extends to the external limit- 



