144 MACROMOLECULAR COMPLEXES 



fixation and embedding procedures, with tlieir concomitant extrac- 

 tion artifacts. 



Since the photopigments form an integral part of each unit disk, 

 it appeared worthwhile to apply the new low-temperature prepara- 

 tion techniques to a comparative study of dark- and light-adapted 

 photoreceptors. Rapid freezing preserves the labile photopigments. 



Fig. 16. External segment of light-adapted guinea-pig retinal rod, showing 

 intermediate lines and compact fine-structure of the unit disks. Liquid helium II, 

 osmium-cryofixation preparation. X600,000. 



Fig. 17. External segment of dark-adapted guinea-pig retinal rod, showing 

 granular and rod-shaped structures (arrows) with indications of orientation in 

 the planes of the unit disks. Platinic chloride, osmium-cryofixation preparation 

 after liquid helium ll-freezing. X600,000. 



and it was therefore hoped to detect characteristic differences in the 

 ultrastructure of dark- and light-adapted rod disks as a basis for 

 further studies on photopigment localization. In preliminary experi- 

 ments (Fernandez-Moran, 1959a), dark-adapted fresh or glycerin- 

 ated guinea-pig retinas were rapidly frozen in liquid helium II or in 

 Freon 22-liquid nitrogen, then subjected to freeze-substitution and 

 low-temperature polymerization. When the frozen retinas were 



