Irvino- Hardestv 



535 



nuclei at this stage all appear nearly similar and are simply imbedded in 

 the common proto])]asm of the syncytium, the amount about a nucleus 

 depending upon its position. 



This study has not involved the spinal cord of the very young stages 

 and. perhaps for that reason, by none of the stains I have employed does 

 the growing axone, Iwfore medullation or at any stage in my prepara- 

 tions, appear as a " group of fibrils," or fibril l)undles, as described by 

 Bardeen for the periplieral nerves and by others cited hy him. The 

 axone of the young spinal cord, at least froui G centimeters np, appears 

 as a nearly homogeneous strand of even calil>er which increases appre- 

 ciably in size with the growth of the speciuien. At best its structure 

 shows nothing more than a fine, elongated reticulum, the threads of 

 which become heavier and more evident as the fil)er approaches maturity. 

 Though Bardeen's observations were confined to the developing peripheral 

 nerves, my failure to note the filiril-gronp form of the young axone must 



Fig. 1. From longitudinal section of white substance of spinal cord of 

 pig of 11 centimeters. Benda method. «= nerve axones; s = syncytium. 

 X 550. 



be due to unsuitable methods, or to the fact that I have not examined 

 the very young stages, for it is hardly probable that tlie axone in the 

 central system is essentially difi'erent from the peripheral axone. Also 

 Bardeen states that as early as 2 centimeters, most of the peripheral 

 (intercostal) nerve fil;)ers of the pig are covered with embryonic myelin. 

 This indicates that myelination occurs very much earlier in the per- 

 ipheral than in the central nervous system of pigs, for not till about 

 IG centimeters have I observed any appearances at all suggestive of the 

 illustrations he gives as representing embryonic myelin. 



Fig. 2 represents the frayed end of a clump of axones from the white 

 substance of the spinal cord of a pig of 16 centimeters teased by the water '«*. 

 method. This material was fixed in the corrosive sublimate and osmic 

 acid mixture and the teased fragments stained with acid fuchsin. The 

 appearance of the interaxone substance was verified from sections of 

 the same stage stained by other methods. The teased preparation is 



