164 



BLIND VERTEBRATES AND THEIR EYES. 



In a fish 25 mm. long the smaller nuclei measure 2.5 \l, the larger ones measure 

 3.5 to 5 /x. In the specimen 123 mm. in length the nuclei measure but 2 to 3 fi. 

 Evidence that the smaller nuclei in the younger specimen are degenerate is fur- 

 nished by the fact that optic fibers can not be traced to the smaller ganglionic 

 nuclei in a 25-mm. specimen. 



The most disorganized eye found is the left one of the largest fish examined, 

 130 mm. long (plate 10, fig. e). The fibrous sheath (sclera) is thick; the cartilage 

 is large, 64 by 96 /x in section. The eye itself is a disintegrated mass abundantly 

 provided with granular pigment and without well-defined outline or structure. 

 The right eye of the same specimen is less degenerate (plate 10, fig. f). It is an 

 elongated vesicle 60 by 256 p in section, with a large cartilage to one side of its 

 distal half, 48 by 160 n in section, and two smaller proximal ones, one of which 

 measures 24 by 32 /1 in section. Associated with the retina of this eye is a struc- 

 ture that I described as a possible lens in my first paper. It consists of a few 

 nuclei about which there are concentric layers of a homogeneous tissue. Consider- 

 ing the fate of the lens in all the young fishes examined, it seems very doubtful, 

 if not impossible, that this structure should be a lens. 



That the eyes of these largest individuals belong to the fourth period is seen in 

 the fact that they become distended vesicles whose parts are finally resorbed after 

 undergoing degenerative changes. The scleral cartilages offer an exception to the 

 general fate. 



Summary of the Origin, Development, and Degeneration of the Eye and its Parts. 



? I do not know. — Does not take place. 



COMPARATIVE RATE OF ONTOGENETIC AND PHYLOGENET1C DEGENERATION 



OF THE PARTS OF THE EYE. 



On pages 134 et seq. an outline of the probable phylogenetic history of the eye 

 of Amblyopsis is given. In the preceding chapter the rate of ontogenetic degen- 

 eration and its extent has been found to vary in different parts of the eye. It has 

 also been found that certain parts begin to degenerate earlier than others. We 

 shall now attempt to discuss briefly the ratio between the rates and extent of onto- 

 genetic degeneration and the rate and degree of phylogenetic degeneration implied 

 by the structure of the eye. The discussion is somewhat intangible, but certain 

 definite results can be obtained by it. 



