LAMELLAR SYSTEMS 145 



examined at approximately —150° C in a special low-temperature 

 stage, the characteristic salmon-pink color of the dark-adapted 

 retinas could be observed essentially unaltered over periods of 

 several hours. However, in the subsequent process of freeze-sub- 

 stitution, the red color regularly changed to a pink-yellow. Upon 

 completion of the low-temperature photopolymerization process, the 

 dark-adapted retinas exhibited a distinct yellow color and could be 

 readily distinguished from the corresponding light-adapted retinas. 

 If ultrathin sections of dark-adapted retinas are collected on inert 

 fluids instead of water and examined in the electron microscope, 

 using a liquid-nitrogen cold stage (Fernandez-Moran, 1959a, 1959d), 

 a characteristic electron-dense constituent is detected associated 

 with the double layers. This component appears to be very sensitive 

 to the effects of the electron beam, and even when a microbeam of 

 very low intensity is used, it rapidly disintegrates. It was therefore 

 found necessary to stain the dark-adapted retinas with platinic 

 chloride and other heavy-metal salts introduced during the process 

 of freeze-substitution. Under these conditions, the dark-adapted 

 retinas exhibited a distinct yellow color which was preserved with 

 little alteration throughout the entire preparation process. Exami- 

 nation of thin sections of these specimens (Fig. 17) disclosed the 

 presence of a granular material, associated with the dense layers and 

 the intermediate line, which seemed to exhibit a certain degree of 

 orientation as compared with the corresponding preparations of 

 light-adapted retinas (Fig. 16). However, these observations are 

 still very preliminary, and much more work remains to be done be- 

 fore definite conclusions can be drawn. The described changes oc- 

 curring in dark-adapted retinas during the process of freeze-sub- 

 stitution emphasize the lability of the photopigments and the need 

 for caLition in the interpretation of these images. Through a corre- 

 lation of the electron microscope studies with polarization-optical 

 analysis of the characteristic dichroism observed in dark-adapted 

 retinal rods (Schmidt, 1951), it may eventually be possible to local- 

 ize the photopigments and establish their structural relationships 

 to the lipoprotein layers. 



The photoreceptor elements of the insect eye, which consist of 

 closely packed, thin-walled tubules approximately 400 to 500 A in 

 diameter (Fernandez-Moran, 1958), have also been examined with 

 the new techniques. The walls of each of the tubular compartments 

 are formed by a thin osmiophilic line, 20 to 30 A wide, associated 



