STEUCTURE OF RETINA — PHRYNOSOMA 



353 



the rod and cone nuclei as occupying a single laj^er. According 

 to Tafani ('83) the same is true in Champsia lucius. 



Cones. The cones of Phrynosoma exhibit a marked variation 

 in size, shape, and structure, as is illustrated in figures 4 and 6; 

 the cones in the former figure are taken from the fovea. Al- 

 though there is considerable variability in the shape and struc- 

 ture of the inner segment of the cones outside of the fovea, all 

 are alike in the possession of an oil drop and a rather long conical 

 outer segment. Cones a, b, c, d, and e (fig. 6) have been taken 

 from a region about 1.5 mm. below the entrance of the optic 

 nerve. Of the different kinds of cones which are typical of this 

 region, types d and e are the most prevalent. In addition to these 



a 6 c d e / g h i 



Fig. 6 Selected cones from the extra-foveal portion of the retina. X 935. 



types, all of which contain a refractive paraboloid, a nmnber of 

 cones (/, g, and K) without paraboloids have been found. Types 

 / and g are quite numerous in the region of the retina just below 

 the entrance of the optic nerve, but do not constitute the sole 

 kind of element in this region since tj-pes a to e are present to a 

 variable extent. Cones/ and g were drawn at a distance of 10 ^ 

 from the entrance of the optic nerve. Tj^pe h (large cones with 

 narrow inner segments and without paraboloids) is infrequent and 

 not characteristic of any particular region. 



In the region of the ora serrata, the cones become considerably 

 shortened. The predominant type of visual cell in this region is 

 illustrated by i (fig. 6), it is characterized by a broad inner seg- 

 ment with an exceedingly short myoid and a larger paraboloid. 



