330 



THE BIOLOGY OF THE FROG 



CHAP. 



portion of the rods. This is followed by a refractive lens- 

 shaped body, beyond which the cone becomes constricted. 



FIG. 91. Sections through the outer part of the frog's retina with its pig- 

 mented epithelium. The figure on the left is taken from a frog which had 

 been exposed to daylight for five hours. The processes of the three outer 

 pigment cells extend inward to the external limiting membrane, and the 

 pigment they contain has streamed inward, surrounding the rods and 

 cones nearly to their base. The cones are strongly retracted, so that 

 their outer segments lie near the limiting membrane. The figure on the 

 right is taken from a frog that had been kept for forty-eight hours in 

 darkness. The pigment in the processes of the pigment cells is drawn 

 back toward the center of the cells, and the cones are extended outward 

 between the rods. The nuclei of the rod and cone cells are shown below 

 the limiting membrane. (From Gaupp, after Van Genderen Stort.) 



The nucleus lies in an enlargement on the inner side of the 

 outer limiting membrane. The inner end of the cone, like 

 that of the rods, runs out into a fiber which enters the outer 

 reticular layer. The cones vary in size and form, as many 



