DEVELOPMENT OF THE EYE OF AMBLYOPSIS. 163 



set in. The cells with smaller nuclei are probably degenerate. In the oldest 

 fish only cells of the second type are found. 



A number of changes take place during the third period, some of which can 

 be classed neither as progressive nor as retrogressive. As the fish grows, the eyes 

 are farther and farther removed from the surface. In the fish 25 mm. long they 

 are nearly 1 mm. below the skin, and in the largest specimen examined they are 

 as much as 5 mm. 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 in- 

 stance, 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 struc- 

 ture, the pigment granules being in many cases entirely absent. Rarely the pig- 

 ment layer changes to a high columnar epithelium. The stages of this period have 

 not been successively observed as in the younger period, and the genetic relation- 

 ship of different stages is not always apparent. 



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

 of about 100 mm. to the end of its life. There are distinct features that charac- 

 terize the eye of this stage (plate 10, figs. c-g). 



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

 stages. The scleral cartilages are as well developed as at any time. 1 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 pig- 

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

 the diameter of the eye in previous stages (plate 10, figs. F and g). This develop- 

 ment of the pigmented layer beyond the requirements of the retina has also been 

 seen 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 F, the rest of the retina forms an insignificant little ball 

 of tissue. In an eye of an individual 105 mm. long whose pigmented epithelium 

 forms a vesicle 320 /x in diameter, the rest of the eye forms a small sphere 60 p in 

 diameter in contact with the iridian part of the pigment (plate 10, fig. g). The 

 elements composing this little ball and representing the retina have also under- 

 gone a marked senescent modification. The optic nerve is no longer evident. 2 

 The ganglionic cells no longer form a compact mass, but are either unidentifiable 

 or irregularly scattered. The cells of the outer nuclear layer are also less regular. 

 While in the second period and up to 95 mm. in length two sorts of nuclei are 

 distinguishable, some of them small and dense, others larger and granular. In these 

 later stages they are all small and dense, no granular ones being present, and their 

 outlines are less well defined than in the young. 



1 In the left eye of a specimen 105 mm. 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. 



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

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



