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BLIND VERTEBRATES AND THEIR EYES. 



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layer exclusive of the twin cone nuclei. But extending just with- 

 out the external limiting membrane a large number of short 

 processes are seen between the cone nuclei (fig. 35 c). Whether 

 these are degenerate rods, I am unable to say. 



The outer nuclear layer differs materially in the younger, i.e., 

 smaller (29 mm.), specimens and in the largest specimens. In the 

 younger specimens it consists of several layers of cells exclusive of 

 the cone cells, which in this case can be counted with the laver of 

 rods and cones. In the larger specimens this is reduced to a single 

 layer of nuclei less densely packed, with occasionally a horizontal 

 nucleus near the base which is less granular, staining a more uni- 

 form color. 



In the largest specimen the outer nuclear layer makes up about 

 7 per cent of the total thickness of the retina, in the smaller 

 specimens it is slightly thicker, forming 10 per cent of the total 

 thickness. 



The outer granular layer differs also materially in the largest 

 and smallest specimens. In the largest it forms a thin layer entirely 

 free from nuclei and with a total thickness of but 2 or 3 /x. 



In a specimen 39 mm. long this layer is 5 /x thick, distinctly 

 granular, contains a few round nuclei — not differing from those 

 of the inner nuclear layer. (See fig. 35 c.) These are probably 

 members of the layer of fulcrum cells. The latter are not separable 

 from the underlying bipolar cells in other regions. In tangential 

 sections they appear in groups of two, but are much fewer in 

 number than the twin cone cell. The inner layer of the inner 

 nuclear layer is composed of distinctly larger cells in the largest 

 specimen and separated from the rest by a slight interval. The 



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Fig. 36. (a) Vertical Section through Retina of Cfwlogasler papilli/erus. (b) Section from Outer Margin of 

 Retina to Base of Cone Bodies, (c) Section through Cone Nuclei, (rf) Section through Basal Segments 

 of Cones, (c) Section through Outer Limiting Membrane, Outer Nuclear Layer with Cone Cell Processes. 

 outer Reticular Layer and Outermost Layer of Outer Nuclear Layer. Nuclei in last layer are frequently 

 in pairs, but do not correspond to and are much less numerous than t\vin cones. (/) Section through Lower 

 Half of Left Eye seen from behind. The eye was depigmented. Oval nuclei nl. I. at ora scrrata. scl., sclera; 

 ch., choroid; i., iris; (g) Lower part of Iris of Zygonectes nolatus. 



