336 Sheath Cells and Axoiie Sheaths in the Central Nervous System 



preferable in that the axones, uncut, may be followed a considerable dis- 

 tance and being often washed clean of the interaxonic syncytium, they 

 may be studied more closely. It is seen that even in the pig of 16 centi- 

 meters, a stage when the medullation of the peripheral nerves is well 

 advanced, most of the axones of the spinal cord show no signs of medul- 

 lation, being but slender threads (a) of more or less even contour, with 

 the substance of the syncytium (s) adhering to them. 



The fibers indicated by b in Fig. 2 show the appearance of the first stages 

 of the accumulation of myelin, or at least the first stages to be observed 

 after the technic here employed. The myelin begins as small globules of 

 various shapes and sizes adhering to the axone, giving it a beaded appear- 

 ance. The globules are but very slightly blackened by the osmic acid 

 at this stage and then upon their surface only, making them appear as 

 small blisters which resist the action of the water in teasing. When 

 washed clean of other adherent substance they may be observed minutely 

 and there is no sign whatever of the presence of any other sheath. Be- 

 tween adjacent blisters and connecting them there is usually discerned 

 a thin film on the axone but not always. Usually the globule appears 

 adhering to one side of the axone rather than evenly surrounding it. 

 This form of the first appearance of the myelin upon the axone is similar 

 to that described by Yignal, Westphal, Wlassak, Kolster, Bardeen and 

 others. 



Those of the observers who take into consideration the sheath of 

 Schwann of the peripheral fibers, give varying accounts of the time 

 of the appearance of the myelin. After examining the great amount of 

 literature upon the subject, it seems that the sheath of Schwann usually 

 appears upon the peripheral axone before the myelin begins to be depos- 

 ited, but often simultaneously with its appearance, and sometimes after 

 the appearance of the myelin. The latter sequence indicates that the 

 sheath of Schwann is not concerned in the origin of the m^-elin. 



The fiber indicated by c in Fig. 2 is an example of the most advanced 

 stages of myelination to be found in the spinal cord of pigs of 16 centi- 

 meters. It is the only fiber found after considerable search through the 

 preparations of this stage which apparently possesses a sheath cell, though 

 the protoplasm of this cell is not distinctly differentiated. There is posi- 

 tively nothing indicating such cells upon fibers of earlier stages of inedul- 

 lation. In sections of specimens of this age stained by the Benda method 

 I have found no cells distinctly clasping the medullating axone, and 

 showing the definite outline and differential staining of those found in 

 the later stages, and especially in pigs of 21 centimeters. Occasionally 

 a nucleus may be seen upon the side of an axone with protoplasm sur- 



