CONCLUSIONS CONCERNING EYES OF SALAMANDERS. 41 



(6) Rods and cones are not found. 



(7) The outer reticular layer has disappeared. 



(8) The inner and outer nuclear layers form one layer of cells indistinguishable 

 from each other. 



(9) The inner reticular layer, as usual in degenerate eyes, is relatively well 

 developed. 



(10) The ganglionic layer is well represented and connected with the brain by 

 the well-developed optic nerve. 



(n) The epithelial part of the iris is at first simple, with an outer pigmented 

 and an inner colorless layer. With age the margins of the iris become folded in- 

 ward in such a way that the pigmented layer may be thrown into folds in the interior 

 of the eye, while the colorless layer is but little affected. 



(12) Pigment granules, and rarely pigmented cells, are associated in the eye 

 with the optic nerve. 



(13) The eye is more degenerate than that of the European Proteus. It is less 

 degenerate than that of the North American blind fishes, Ainblyopsis, Typhlichlhys, 

 and Troglichthys, but much more so than that of the species of Chologastcr. 



SUMMARY IN REGARD TO TYPHLOTRITON. 



(1) Typhlotriton is an incipient blind salamander living in the caves of south- 

 western Missouri. 



(2) It detects its food by the sense of touch without the use of its eyes. 



(3) It is stereotropic. 



(4) Its eyes show the early stages in the steps of degeneration from those of 

 salamanders living in the open to those of the degenerate Typhlomolge from the 

 caves of Texas. The lids are in process of obliteration, the upper overlapping the 

 lower so that the eye is always covered in the adult. The sclcra possesses a car- 

 tilaginous band in the larval stages but not in the adult. The disappearance of 

 the cartilage is probably an incident of the metamorphosis, not of the degeneration 

 the eye is undergoing. The lens is normal. The retina is normal in the larva with 

 a proportionally thicker ganglionic layer than in the related epigean forms. 



(5) Marked ontogenetic degenerations take place during and shortly after the 

 metamorphosis, (a) The outer reticular layer disappears, (b) The rods and 

 cones lose their complexity of structure, such as differentiation into inner and outer 

 segments, and finally are lost altogether. 



