188 THE EYES OF THE BLIND VERTEBRATES OF NORTH AMERICA. 



are as much as 5 millimetres beneath the surface of the skin. The scleral cartilages 

 develop progressively probably during the entire period, in some cases encroaching 

 on the regular outline of the eye. Other processes which are progressive nevertheless 

 do not tend to make the eye a more perfect organ of vision. The pupil, for instance, 

 becomes closed in many cases, or reduced to a very minute opening. The vitreous 

 cavity, which was still evident, becomes, concomitantly with the closing of the pupil, 

 entirely obliterated. The pigmented layer becomes a variable structure, the pig- 

 ment granules being in many cases entirely absent. Rarely the pigment layer 

 changes to a high columnar epithelium. The stages of this period have not been suc- 

 cessively observed as in the younger period, and the genetic relationship of different 

 stages is not always apparent. 



4. The Fourth Period. — This extends from the time the fish has reached a length 

 of about 100 millimetres to the end of its life. There are distinct features that char- 

 acterize the eye of this stage (PL XII, Figs. 6-10). 



The fibrous capsule enveloping the eye is distinctly thicker than in younger 

 stages. The scleral cartilages are as well developed as at any time.* 



The eye-muscles, as far as present, show no indication of degeneration, and their 

 striation can readily be made out in all individuals. 



The most marked changes take place in the size of the eye itself. The pigmented 

 layer becomes distended to form a thin-walled vesicle of two or three times the diam- 

 eter of the eye in previous stages (Figs. 7, 10). This development of the pigmented 

 layer beyond the requirements of the retina has also been observed in the eyes of 

 Rhineura and other blind vertebrates. The cells of this layer become spherical or 

 attenuated and the columnar epithelium converted into a thin epithelium thickened 

 in places. Within this vesicle, whose sides may be compressed as in Figure 10, the 

 rest of the retina forms an insignificant little ball of tissue. In an eye of an individual 

 105 millimetres long whose pigmented epithelium forms a vesicle 320 micra in diameter 

 the rest of the eye forms a small sphere 60 micra in diameter in contact with the iridian 

 part of the pigment (Fig. 7). The elements composing this little ball and representing 

 the retina have also undergone a marked senescent modification. The optic nerve 

 is no longer evident. f The ganglionic cells no longer form a compact mass, but are 

 either unidentifiable or irregularly scattered. The cells of the outer nuclear layer are 



* In the left eye of a specimen 105 millimetres long no cartilages were found. It is not possible to say 

 whether they had disappeared or were never developed. Because of the irregularity in the development of 

 these cartilages and their large size in other individuals of this period, I am inclined to think cartilages never 

 appeared in this specimen. 



t The optic nerve can be traced as a very delicate filament through the pigment layer in an individual 123 

 millimetres long. In this eye the choroid fissure was still open. 



