No. i.] BLL\n VERTEBRATES OF NORTH AMERICA. 37 



to not more than three series of cells. Aside from the differ- 

 ences noted above, the eye of the larvae of Typhlotriton is 

 apparently normal in all of its histological details. This rela- 

 tive thickness in the different sizes of the larvae may be gath- 

 ered from Figs. 2-5 and from the comparative table at the 

 end of the paper. 



Figs. 2-5 are drawn with the same magnification and show 

 the relative thickness of the different layers in the retinae 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 Amblystoma more 

 than the young does. The inner reticular layer is compara- 

 tively 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. 5.) 



The inner nuclear layer consists of about seven series of cells 

 in the smallest larva and of four to seven in the largest. The 

 cells in the preparations available cannot be separated into bipolar 

 and spongioblastic layers, nor are horizontal cell layers distin- 

 guishable. 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. 9), 

 but none of its structure remains. It is fair to suppose that 

 the fibers forming this layer are resorbed during the meta- 

 morphosis. This layer seems to be the very first obliterated by 

 the processes of degeneration both ontogenic and phylogenic 

 in this as in other vertebrates 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 /x. The relative sizes and 



