Slonaker, Eye of the Mole. 357 



an average length of .0178 mm. and an average diameter of 

 .0007 mm. It is almost uniform in diameter from its base to 

 the abrupt tapering into the thread-like external segment. The 

 external segment has an average length of .0088 mm. It is 

 very delicate and resembles a thread or line. It extends out- 

 ward in a more or less wavy manner and is lost among the 

 pigment cells. The base of the rods lies near the outer 

 boundary of the outer nuclear layer. In fact the bases of the 

 rods frequently lie between the nuclei. A delicate process 

 extends from the base of the rod to its nucleus (r;/). This 

 process is of various lengths depending on the position of the 

 nucleus within the nuclear layer. 



The cones are more conspicuous than the rods. They 

 have a total average length of .0178 mm. There is more 

 variation in their length than in the rods. They are found to 

 vary from .0143 mm. to .0220 mm. Like the rods, they can 

 easily be divided into the broad internal segment and the thread- 

 like external segment (Fig. 30, ^). The former has an average 

 length of .0107 mm. and the latter .0071 mm. The internal 

 segment has a greater variation in length than the external seg- 

 ment. It varies from .0085 mm. to .0142 mm. This portion 

 of the cone is more or less cone-shaped, the base being wide 

 and in direct contact with its nucleus. The average diameter 

 of the base is about .0021 mm. It tapers rapidly to a diameter 

 of .0014 mm. which it maintains for a short distance. It then 

 decreases rapidly in diameter to the thread-like external seg- 

 ment. The bases of the cones do not lie in the same level. 

 This is due to the fact that their nuclei seem to have been push- 

 ed sometimes inward and sometimes outward from their natural 

 position. Those cones whose nuclei are most deeply located 

 in the outer nuclear layer are the longest. The shortest cones 

 are associated with the nuclei lying farthest out. The result is 

 that the external segments of all cones lie in almost the same 

 level. 



The rods and cones stain in a normal manner and in gen- 

 eral closely resemble elements in the normal mammalian eye. 



The nuclei of the outer nuclear layer very closely resemble 



