The Structure of the Retinal Receptors of the Toad Eye 



195 



Fig. 2. Longitudinal section of a proximal cell outer segment 

 in a double cone. The double membrane disks are well 

 demonstrated. Magnification 58,000. 



In the double cones we may distinguish between 

 one outer segment with a more proximal location 

 than the other. The proximal outer segment is larger 

 than the distal one and reminds of a rod outer seg- 

 ment. It is, however, lacking the incisions character- 

 istic for the rod outer segment. The distal outer seg- 

 ment resembles that of the single cones. 



In the single cones the dimensions of the double 

 membrane disks are: total thickness 230 A, base 

 diameter 2-3 //. The osmiophilic layers are 70 A 

 thick and the interposed, less osmiophilic layer 95 A 

 thick. The average spacing is 490 A. 



In the proximal cell of the double cones the dimen- 

 sions of the double membrane disks are: total thick- 

 ness 130 A, base diameter about 2 //. The average 

 spacing is 220 A (fig. 2). 



In the /////('/• segments of the single cones a strongly 

 osmiophilic oil droplet may occur. It is almost 

 completely surrounded by mitochondria. In some of 

 the mitochondria located in contact with the oil 

 droplet a system of 8-10 concentrically oriented and 

 very densely packed double membranes may be 

 observed. 



The proximal component of the double cones 

 contains distally in its inner segment an ellipsoid 

 consisting of mitochondria. Proximally to the ellip- 

 soid the well delimited paraboloid is observed. It 

 consists of a homogeneous ground substance in 

 which vacuoles and vesicles are present (fig. 3). 



The cytoplasm of the distal component proximal 

 to the ellipsoid contains irregularly outlined vacuoles 

 of varying sizes. No paraboloid is present. The inner 

 segments of the two components of the double cones 

 are in close contact with the osmiophilic layer of 

 their plasma membranes separated by a less osmio- 

 philic interspace the thickness of which varies from 

 60 to 240 A (fig. 4). This arrangement is similar to 



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 ^ 



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t 



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A: 



Fig. 3. A double cone. The inner segment of the proximal 

 cell and part of the inner segment of the distal cell. Notice 

 the ditVerence of the cytoplasm in the two cells. Magnifica- 

 tion X 5600. 



that of the plasma membranes at retinal synaptic 

 contacts (6). 



Electrophysiologic studies by Svaetichin (8) per- 

 formed on twin cones in the fish retina have given 

 support to the interpretation of twin cones as 

 representing a system which is stimulated by two 

 complementary colors. The contact surface between 

 the inner segments has been interpreted by Svae- 

 tichin to act in a similar way as a synapse. The 

 morphology of the double cones of the toad retina 

 makes such an assumption quite justifiable from a 

 morphologic point of view. 



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" f 



V " .^ 'l/M 



n 



Fig. 4. The plasma membranes between the cells in a double 

 cone. To the left the proximal cell with a part of its mitochon- 

 dria aggregation and the paraboloid structure. Magnifica- 

 tion 58,000. 



