334 Sheath Cells and Axoiic Sheaths in the Central Nervous System 



sheaths present, and further, hy the conclusion of Gurwitch that for the 

 peripheral nerve fibers the sheath of Schwann has nothing in common 

 with the medullary sheath. It has also been shown by Kolster and 

 Bardeen that the myelin may begin to appear about the axone before the 

 sheath of Schwann is evident, though, as a rule, the axone of the per- 

 ipheral fiber is enclosed by the sheath of Schwann before the formation 

 cf the myelin is apparent. 



6. The medullary sheath is composed of at least two parts, first, the 

 myelin (lecithin, etc.), and second, its su})porting framework. As early 

 as 1862 Mauthner roughly depicted this framework for certain giant 

 fibers of the trout and the coarser portions of it were observed in 1876 by 

 Ewald and Kiihne. who gave it the name " neurol-craiin'' suggesting it 

 to resemble horn in that they found it to resist the action of certain 

 digestive ferments. Since then it has been studied in greater anatomi- 

 cal detail by various investigators, more recently by Wynn and Ilatai, 

 whose observations were also made upon peripheral nerve fibers. 



The seal-ring cells which I have observed in the spinal cord of the 

 pig are somewhat puzzling both as to their origin and their function. In 

 the first place, they appear to have a period of maximum abundance. In 

 the spinal cord of pigs of about 21 centimeters they are more easily found 

 than at any other stage I have examined. In transverse sections stained by 

 the Benda neuroglia method, I have seen as many as three evident in a 

 single field of the oil immersion. While often a field contains none at all, 

 it usually requires but little search to find one, though the nucleus, situ- 

 ated in the thicker side of the ring, may not always be contained in the 

 section. In the older fcetus they seem less abundant, and in the suckling 

 pig it becomes difficult to find them in sections, while in sections of the 

 adult spinal cord satisfactory examples of them are even more seldom 

 found. Their protoplasm seems to have been used up and a free nucleus 

 clasping the side of a nerve fiber nuiy possibly be a neuroglia nucleus 

 instead. 



There are no direct indications of seal-ring cells prior to medul- 

 lation. Xone have been observed upon fibers in the earliest stages of 

 medullation. The conditions before the accumulation of myelin has 

 liegun are represented in Fig. 1, which is taken from the frayed end of a 

 longitudinal section of the future white substance of the cord of a pig 

 of 11 centimeters. The axones (a) appear as well-defined threads much 

 smaller than in the older stages and separated by or imbedded in the 

 general protoplasmic syncytium (.9) which is the early form of the 

 neuroglia tissue, no masses of Avhich show diiferential blue staining l)y the 

 Benda method, nor anv definite outlines indicatinu' individual cells. The 



