114 MACROMOLECULAR COMPLEXES 



membrane organization in the living state because they are com- 

 posed ahiiost exckisively of numerous unit membranes multiply 

 wrapped or folded in highly ordered complexes. These compact 

 aggregates of oriented membranes are well suited for analysis by 

 polarized light and x-ray diffraction, which can often be performed 

 on the intact components under physiological conditions (Schmitt, 

 1944; Schmittei«/., 1935). 



In the embryogenesis of peripheral nerve mvelin, Geren and 

 Schmitt ( 1955 ) have shown that the Schwann cells "wrap them- 

 selves" many times around an outgrowing axon by a continuous in- 

 folding of the outer Schwann-cell surface membrane ( Schmitt, 1959, 

 p. 460). After numerous double membranes are thus spirally 

 wrapped about the axon, the layers become condensed to form the 

 compact myelin sheath (Geren and Schmitt, 1955; Robertson, 1959), 

 which exhibits strong birefringence (Schmidt, 1937), indicating a 

 high degree of orientation of the constituent lipid-protein units, and 

 other physical properties characteristic of the smectic fluid-crystal- 

 line state (Schmidt, 1937; Schmitt, 1959). 



The analysis of the myelin sheath ultrastructure is a classic dem- 

 onstration of the advantages inherent in an approach combining 

 direct and indirect methods (Schmidt, 1937; Fernandez-Moran, 

 1959b). Based on the polarized-light investigations (Schmidt, 1937; 

 Schmitt, 1944) begun over a century ago, and on subsequent low- 

 angle x-ray diffraction studies (Schmitt et al., 1935), the concentric 

 laminated structure of the sheath was accurately predicted, and the 

 thickness of the layers deduced, before the electron microscope 

 provided direct confirmation of the postulated multilayer arrange- 

 ment (Fernandez-Moran, 1950; Robertson, 1959). 



Electron microscopy is the only method that permits direct 

 visualization of structural details of molecular dimensions within a 

 selected region and thus furnishes data well above the level of 

 statistical uncertainty commonly associated with indirect analytical 

 methods. However, electron microscope investigations are severely 

 limited by numerous preparation artifacts resulting from complex 

 perturbations of the living state when biological systems are sub- 

 jected to fixation, dehydration, embedding, and ultrathin-sectioning 

 procedures, and finally to the deleterious effects of high-vacuum and 

 electron-beam irradiation during observation. 



Practically all of our present direct knowledge of lamellar sys- 

 tems, and of cellular fine structure in general, is based on electron 



