'o4'i .Shoalh Cells and A.xono Sheaths in the Central Nervous System 



adhere so closely to the myelin as to he invisihle and to break with 

 the breaking- of the myelin sheatb. This thin membrane-like appear- 

 ant'e was first noted Iiy l\anvier in the cord of the dog and be referred to 

 it as a membrane. It bas since been discussed by Schiefferdccker and 

 others and its existence has been repeatedly denied. 



The nerve corpuscles or Schwann's corpuscles of the peripheral nerve 

 fibers are descrilied as lying under the sheath of Schwann — between it and 

 the myelin sheath. The cells here observed upon the fibers of the spinal 

 cord seemingly lie upon the medullary sheath, being attached or in some 

 way in close relation to it, without being enclosed upon it by a percep- 

 tible membrane. Sometimes in the teased preparations the protoplasm 

 of the cells seems to blend into the blackened myelin (e, Fig. 5), but in 

 the stained sections the protoplasm appears distinctly outlined from the 

 myelin. The latter is perhaps the true condition, for whatever the func- 

 tion of the cells, the granular, more deeply staining portion probably 

 only represents ' the unti'ansformed endoplasm of the cell. In Fig. 7, 

 sc, is shown one of the seal-ring cells as found in transverse sections of 

 the spinal cord of the adult hog when stained by the Benda method. It 

 is merely a nucleus practically free of endoplasm and its shape and posi- 

 tion are the only features which suggest its being one of the cells in 

 question. 



I am as yet unable to reach a definite solution of the exact significance 

 of these cells. If one follows them through the preparations with the 

 idea that they are a distinct type of cell, having probably to do with the 

 development of some part of the medullary sheath, the following observa- 

 tions may be made in support of this idea : 



1. They do not appear ditferentiated till after the acquisition of mye- 

 lin has begun. 



2. When first indicated they do not appear as definitely outlined and 

 differentiated cells, but rather their more deeply staining protoplasm 

 seems to grade off into that of the general syncytium and to be continuous 

 with it. This, and their being attached to a medullating fiber suggests 

 that they are derived from nuclei and protoplasm formerly a part of the 

 syncytium, and that their differentiation may be due to influences ex- 

 erted upon it by the developing myelin. 



3. During the period in which the process of myelination is at its 

 height, they appear as distinctly differentiated cells with considerable 

 protoplasm (or probably endoj)lasm) about their nuclei, often sulficient to 

 completely encircle the growing sheath at the level of the nucleus. 



-i. With the further growth of the medullary sheath, the protoplasm or 

 endoplasm of the cells is apparently used up gradually, till as the sheath 



