218 BLIND VERTEBRATES AND THEIR EYES. 



not pigmented and forms a distinct ciliary process. Between the latter and the rest 

 of the retina there is an accumulation of elongate nuclei. 



This retina has reached a stage in an irregular process of histogenesis, or it 

 has earlier stopped at such a stage of differentiation, or finally, it has reached its 

 present condition as a degeneration from an earlier, more highly differentiated 

 stage. From the material at hand it is impossible to determine when the ret- 

 ina reaches its highest stage of development and when it begins to degenerate. 



A slightly higher stage of differentiation is found in one of the eyes of one of the 

 unborn young of 76. In this eye, the retina has about the same total thickness. 

 There is found in places a very distinct separation of the outer layer of nuclei into 

 an inner layer, a reticular layer, and an epithelial layer. To one of the epithelial 

 nuclei a cone is found attached (plate 24, fig. e). A ciliary process is not seen in 

 this eye nor in the group of elongate nuclei so conspicuous in the younger stage. 

 The inner layer of the uvea, as well as the outer, is pigmented. 



Beyond birth only general processes can be described without entering into a 

 minute description of each eye. The retina degenerates progressively and it 

 seems to do this accompanied by one of two modifications in the general structure 

 of the eye. The eye may shrivel (plate 20, figs. B, c; plate 21, fig. a), the pig- 

 ment layer lying close against the rest of the retina ; or the pigment layer may sepa- 

 rate itself from the rest of the retina and become very greatly distended, the retina 

 itself forming but a small segment of the eye vesicle (plate 22, fig. a; plate 24, figs. 

 A, c). Plate 22, figure A, represents such an eye, in which the retina is well con- 

 tracted and the pigment layer shriveled. The optic nerve passes through the 

 vesicle. The beginning of such a modification is probably to be seen in plate 21, 

 figure b. In other cases the retina is drawn out laterally (plate 24, figs, a, c). 

 Such vesicular eyes were also found in old individuals of Amblyopsis. There does 

 not seem to be any increase in the amount of pigment, and, since it is scattered 

 over a larger area, the pigmented layer of these vesicular eyes is less densely pig- 

 mented than that of the shriveled eyes. In one eye conditions normal to a fish eye 

 are more nearly retained. 



I am not able to say that one part of the retina undergoes a more rapid 

 degeneration than another. They all reach the vanishing point with extreme 

 old age. 



In an old individual (94 mm.) the eye of one side consisted of a few vacuoles 

 surrounded by nucleated fibrous tissue (plate 24, fig. d). It is impossible to deter- 

 mine to what these parts of the eye belonged. There are also scattered pigment 

 granules and cells, while near this eye are a few pigment masses. The eye of the 

 other side is better preserved and represented in plate 23. In one eye, which is 

 shriveled to very small dimensions, a peculiar lenslike structure occupies most of 

 the interior. Such lenslike structures I found in Amblyopsis and erroneously 

 considered them the lens. In Rhineura it is distinctly seen that the structure fills 

 an invaginated pocket of the pigment layer. 



A census of a series of eyes of individuals from the time of birth to old age gives 

 us the following statistics concerning the lens, the vitreous space (that is, between 

 retina and iris), and the aqueous space (between iris and cornea) : 



