THE EYES OF THE BLIND VERTEBRATES OF NORTH AMERICA. 189 



also less regular. While in the second period and up to 95 millimetres 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. 



In a fish 25 mi l limetres long the smaller nuclei measure 2.5 micra, the larger ones 

 measure 3.5 to 5 micra. In the specimen 123 millimetres in length the nuclei meas- 

 ure but 2 to 3 micra. Evidence that the smaller nuclei in the younger specimen are 

 degenerate is furnished by the fact that optic fibres cannot be traced to the smaller 

 ganglionic nuclei in a 25-millimetre specimen. 



The most disorganized eye found is the left one of the largest fish examined, 130 

 millimetres long (PL XII, Fig. 9). The fibrous sheath (sclera) is thick; the cartilage 

 is large, 64 by 96 micra 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 (Fig. 10). It is an elongated vesicle 

 60 by 256 micra in section, with a large cartilage to one side of its distal half, 48 by 160 

 micra in section, and two smaller proximal ones, one of which measures 24 by 32 

 micra in section. Associated with the retina of this eye is a structure 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. Considering 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 this 

 general fate. 



XIII. THE COMPARATIVE RATE OF ONTOGENIC AND PHYLOGENIC 

 DEGENERATION OF THE PARTS OF THE EYE. 



In my first paper (Eigenmann, '99) I gave an outline of the probable phylogenic 

 history of the eye of Amblyopsis. In the present paper the rate of ontogenic 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 ontogenic 

 degeneration and the rate and degree of phylogenic degeneration implied by the struc- 

 ture of the eye. The discussion is somewhat intanglible, but certain definite results 

 can be obtained by it. 



