338 Sheath Cells and Axone Sheaths in tlie Central Nervous System 



21 centimeters there are still many fibers in the spinal cord which have 

 not ac(iuired a myelin sheath {a). The syncytial protoplasm is less 

 abundant probably because it is being transformed into neuroglia fibers 

 (n) which begin to appear at this stage. 



After birth cells of the seal-ring type are less numerous than in the 

 pig foetus. Fig. 4, c, shows one as found in the suckling pig of two 

 weeks. It is upon a larger fiber than those in Fig. 3 and the protoplasm 

 of the cell is relatively less in amount and merely forms a crescent about 

 the medullary sheath. The nucleus represented by e of this figure is 

 probably a nucleus of a seal-ring cell which has no blue staining proto- 

 plasm about it. This is only inferred from its position, resting upon 

 and here slightly indenting the medullary sheath. It may possibly be 



c 

 n 





p©^ 



'H • 









He 





Fig. ;5. Fig. 4. 



Fig. 3. Areas from transverse sections of spinal cord of pig of 21 centi- 

 meters. Benda method, c and d^ seal-ring cells; a = axones before medulla- 

 tion; n = beginning neuroglia fibers; n« = neuroglia nuclei. X 550. 



Fig. 4. From spinal cord of suckling pig of two weeks. Showing stage 

 more advanced than Fig. 2. Otherwise same as Fig. 2. c^ seal-ring cell; e = 

 probably nucleus of former seal-ring cell (taken from a different field); nz= 

 neuroglia fiber; «C:= neuroglia cell. X 550. 



one of the larger " free " neuroglia nuclei which has acquired this posi- 

 tion. At this stage neuroglia nuclei are sometimes observed which have 

 an area of more compact protoplasm about them and which stains darker 

 than the, now scarce, granular protoplasm of the syncytium. One of these 

 " neuroglia cells " is shown in the figure (nc) and such are described in 

 many of the papers dealing with the neuroglia. Both this and the con- 

 dition represented in the nucleus e are found in the adult material (see 

 Fig. 7, sc). 



In Fig. 5 are arranged some types of fibers selected from teased prepa- 

 rations of the spinal cord of the 31-centimeter pig fixed in the osmic 

 acid mixture and counterstained with fuchsin. Three of these pieces of 



