146 bulletin: museum of compakative zoology. 



similar to the medullary sheath of ordinary nerve-fibres, but is alwaya 

 much thinner. As there is no sheath of Schwann, this medullary 

 sheath must be a secretion of the axis-cylinders which make up the fibre. 

 AVlassak ('98) and Kolster ('99) have recently shown that likewise in 

 the ordinary nerve-fibre the medullary sheath is produced by the axis- 

 cylinder. 



In addition to this medullary sheath there may be found in many 

 species a loose membranous sheath surrounding the fibre in the anterior 

 part of its course through the mesocoele, but never extending caudad 

 into the fourth ventricle. It is apparently formed by the reflection 

 upon the fibre of the membrana limitans interna which lines the ven- 

 tricle. It is in no way similar to Schwann's sheath of the ordinary 

 nerve fibre, which has recently been claimed by Gurwitsch (:00) to be 

 formed from mesodermal cells. 



The central portion of Reissner's fibre resembles the axis-cylinder in 

 its reactions with stains, and under high powers has in section a 

 minutely punctate appearance ; under favorable conditions and by 

 careful focussing this may be seen to be the effect of the cut ends of 

 the axis-cylinders. 



d. Size. The diameter of the fibre remains fairly constant in its 

 course through the brain ventricles and the anterior part of the canal, 

 but diminishes in its middle and posterior portions. Throughout the 

 course of Reissner's fibre one may occasionally see fine fibrillae coming 

 off from it and running toward the periphery of the canalis centralis 

 (Plate 9, Figs. 66, 67). These have been seen both in transverse 

 and longitudinal sections, but as they are very small, usually hav- 

 ing a diameter of 0.1 micron or less, it is difficult to make much 

 out of them. They are largest and most numerous in the postei-ior 

 part of the canal, and at the extreme posterior end can be made out 

 most clearly (Plate 3, Fig. 22). The diminution in the size of Reiss- 

 ner's fibre at its posterior end is probably due to the giving off of fine 

 processes of this sort. In some instances these processes have been 

 seen to pass between the epithelial cells lining the central canal into 

 the tissue of the cord. 



The diameter of Reissner's fibre varies in different groups, reaching 

 its maximum in the teleosts and selachians. ^Vithin a given gi'oup it 

 varies with the size and activity of the species. Among individuals of 

 a species it varies with the size and age. 



The following measurements of the diameter of the fibre and lumen of 

 the canalis centralis show something of these relations. The measure- 



