58 



BLIND VERTEBRATES AND THEIR EYES. 



that a loose tissue was visible, consisting probably of the limiting membrane and 

 ends of the Miillerian fibers. The outer nuclear layer in the adult consists of a single 

 row of nuclei, with a mass of quite homogeneous material about them. This part 

 of the sensory epithelium measures 0.018 mm. The cones are pear-shaped bodies 

 with the smaller end pointing outward, and at intervals of every four or five a 

 shorter one occurs. Each element is differentiated into two parts. By the iron 

 hsematoxylin process of staining, the outer small end is densely stained, while the 

 body of the element is a light granular mass (fig. 21 a). 



The pigment layer (1) is a continuous layer of even thickness, similar in every 

 respect to that of the garter snake. 



One young specimen, 10 cm. in length, was examined. The eye as a whole, 

 as well as the lens, is nearly spherical. The eye measures in width 0.290 mm. 

 and 0.322 mm. in depth. All parts are so developed that the vitreous cavity is 

 relatively much smaller than that of the adult. The coats are thicker, the ciliary 

 processes better developed, the lens capsule thicker, and the retina at the back 

 actually measures one and two-thirds the depth of the adult retina. The ele- 

 ments of each layer are much more numerous than in the adult, and they are 

 packed much more closely together (fig. 216). The ganglion nuclei are apparently 

 arranged one against the other. In the inner reticular layer occur the "interpolated 

 cells." These were not found in the sections of the adult eye that were examined. 

 The cells of the inner nuclear layer are smaller and arranged in five or six rows. 

 There is a well-developed outer reticular layer similar in its make-up to the inner 

 reticular. Instead of a single row of cone nuclei with its surrounding homogeneous 

 mass, as in the adult, this layer in the young consists of five or six rows of small 

 closely arranged cells. The cones likewise are smaller and more numerous (fig. 

 21 b). 



Comparative Measurement of Retinal Layers in millimeters. 



Relative Proportions of Eye Parts. 



