EYES OF TYPHLOTRITON FROM MISSOURI. 39 



is in contact with the lens. In the larva 90 mm. long this layer has been reduced 

 to not more than 3 series of cells. Aside from the differences noted above, the 

 eye of the larval Typhlotriton is apparently normal in all of its histological details. 

 The relative thickness in the different sizes of the larvas may be gathered from 

 figures 10 a to d and from the comparative table at the end of this chapter. 



Figures 10 a to/ are drawn with the same magnification and show the relative 

 thickness of the different layers in the retinas of the larvae of different sizes and of 

 the adult. The adult retina is reduced in thickness by the absence of the rods and 

 cones and the (partial ?) atrophy of the outer reticular layer and by the thinning of 

 the ganglionic layer. The ganglionic layer in the adult contains from two to five 

 rows of cells. In this respect, the adult approaches the condition found in the 

 Amblystoma more than the young does. The inner reticular layer is comparatively 

 thick, that of the young being thicker than that of the adult. 



In the adult the inner nuclear layer is continuous with the outer nuclear layer. 

 (See fig. 10/.) 



The inner nuclear layer consists of about 7 series of cells in the smallest larva 

 and of 4 to 7 in the largest. The cells in the preparations available can not be 

 separated into bipolar and spongioblastic layers, nor are the horizontal cell layers 

 distinguishable. The outer reticular layer is well differentiated, but quite thin in 

 the larvae, and is irregular in outline, adapting itself to the overlying nuclei which 

 encroach on its outlines. In the adult this layer is indistinguishable by the same 

 methods that make it conspicuous in the larva. In places there appeared an open 

 space where the outer reticular layer should be (fig. 10 h 4), but none of its structure 

 remains. It is fair to suppose that the fibers forming this layer are resorbed during 

 the metamorphosis. This layer seems to be the very first obliterated by the pro- 

 cesses of degeneration both ontogenetic and phylogenetic in tins as in other verte- 

 brates with a degenerating eye. 



The greatest change during and shortly after metamorphosis takes place in the 

 layer of the rods and cones. In the larva 35 mm. long, from the mouth of Rock- 

 house Cave, the rods reach an extreme length of 50 ft. The relative sizes and 

 number of these as compared with the much smaller cones may be gathered from 

 figure 12. 



In the larva 90 mm. Jong the outer segments of the rods are much shorter and 

 stain less conspicuously than in the younger. The nuclei of the outer nuclear layer 

 are distinctly in 2 layers, whereas in the younger 

 specimen they are in 3 less regular layers. The cones A 

 are correspondingly fainter than in the young. It (p 

 is surprising that whereas in the larva 90 mm. long 

 we find the rods and cones well developed, they have Fio it 

 greatly degenerated or practically disappeared in the j^f^&^SSSSS? 



adult only a few mm. longer. In an adult specimen 



97 mm. long the rods have retained their normal shape and position, but no 

 differentiation into inner and outer segments was detected. In longer ones most 

 of the nuclei of the outer series have become rounded at both ends. But one cone 

 was found in eyes of the adult over 100 mm. long. It is shown in figure 11 a. In 

 an adult specimen 103 mm. long filmy rods are still evident. They appear as 

 conical spaces above the nuclei free from pigment rather than as possessing any 



