LAMELLAR SYSTEMS 



143 



tion to the general structural pattern, are now seen to be invested, 

 through the presence of an intermediate line, with the same feaures 

 common to all types of compound unit membranes (Robertson, 

 1959). These findings have been further confirmed by cryofixation 

 of other types of vertebrate and invertebrate photoreceptors, using 



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Fig. 14. High-resolution electron micrographs of rod outer segments from 

 light-adapted guinea-pig retina frozen in liquid helium II, showing intermediate 

 lines (arrows) and compact fine-structure of unit disks which are not observed 

 in standard osmium-fixation preparations (see Fig. 15). (a) X600,000. (b) 

 X 350,000. 



Fig. 15. External segment of light-adapted guinea-pig retinal rod, showing 

 profiles of comparatively "empty" unit disks without intermediate lines, as 

 commonly observed in standard osmium-fixed preparations. X400,000. 



a wide variety of heavy-metal salts and other fixation procedures 

 ( Fernandez-Moran, 1959a). Although the possibility of artifacts 

 cannot be discoimted here, it nevertheless appears likely that the 

 low-temperature preparation techniques, with their absence of ex- 

 traction, and other favorable features, reveal a structural array which 

 approximates the natural state more closely than do the standard 



