138 MACROMOLECULAR COMPLEXES 



but it is conceivable that the expansion of the layers may result from 

 association of the glycerol with the water layers at the aqueous 

 interfaces of the fundamental repeating unit of myelin. Although 

 the glycerol effects on the myelin sheath are not completely revers- 

 ible, this may still be compatible with partial recuperation of physio- 

 logical activity. Thus Pascoe ( 1957 ) has shown that the rat's isolated 

 superior cervical ganglion preparation could be protected from 

 damage by cooling to temperatures as low as —76° C, as judged by 

 the size of the transmitted action potential. Glycerol ( up to 20 per 

 cent) progressively slowed nervous conduction, but the effect was 

 reversible (Pascoe, 1957). 



Bromine Cryofixation. In contrast to the osmium-cryofixation 

 preparations (Fig. 10), treatment with iodine or bromine reveals 

 the presence of a more compact, granular multilayered structure 

 (Fig. 11) with a regular period of approximately 50 to 60 A. Al- 

 though this periodicity seems to result from enhancement of the 

 intermediate line, as already noted in nerve fixed with KMn04 

 ( Fernandez-Moran and Finean, 1957), there appears to be an addi- 

 tional staining of the less dense intermediate layer which is normally 

 not brought out in osmium-fixed preparations. In this connection it 

 is of interest to point out that, on the basis of his comprehensive 

 studies of cell membrane structures, Robertson (1959) has postu- 

 lated the possible existence of a polysaccharide monolayer incor- 

 porated in the outer layer of the Schwann-cell surface membrane 

 which contributes to the formation of the intraperiod line in com- 

 pact myelin. Application of more specific reagents may eventually 

 permit reliable localization of this highly hydrated, polysaccharide 

 component. 



Fine Structure of the Myelin Layers. Previous investigations 

 (Fernandez-Moran, 1950, 1952; Fernandez-Moran and Finean, 

 1957) had revealed that the dense myelin layers dissociate into 

 granular or rod-shaped particles after a wide variety of treatments, 

 including enzymatic digestion, KMnOj fixation, and freezing and 

 thawing of nerve. X-ray diffraction studies (Fernandez-Moran and 

 Finean, 1957) also furnished indirect evidence for the presence of 

 a regular organization within the plane of the layers. In vitro nerve 

 degeneration studies (Fernandez-Moran, 1959b) likewise disclosed 

 various forms of granular dissociation of the lamellae closely re- 



