184 THE EYES OF THE BLIND VERTEBRATES OF NORTH AMERICA: 



XL THE HISTORY OF THE OPTIC NERVE. 



The details of the formation of the optic nerve have not been followed. No indi- 

 cations of it were seen in the eyes of the embryos 4.4 millimetres long. In the eyes 

 of embryos 5 millimetres long it is well developed, forming a solid strand of fibres 12 

 micra in diameter which is readily traceable to the brain. The optic nerve increases 

 but little, if any, after its formation. Its development was rapid. In subsequent 

 stages it was not always traceable from all the cells forming the ganglionic mass. In 

 the 6-millimetre larvse its fibres were distinctly traceable from the cells nearest the 

 choroid fissure, while in later stages they were more distinctly traceable from the 

 distal cells of the ganglionic group. The optic nerve can be followed to the brain in 

 all the larval stages and in the young fish up to 25 millimetres in length (PI. XII, Fig. 

 4). The optic nerve is evident within the eye in older stages up to about 100 milli- 

 metres; in the very oldest ones it could not be found. In individuals much more 

 than 25 millimetres long it was not possible to follow the nerve to the brain, though 

 it could usually be followed for some distance from the eye. 



The fibres are never medullated, and I have not so far been able to give them a 

 differential stain. 



XII. THE HISTORY OF THE DEVELOPMENT, MATURITY, AND 

 DEGENERATION OF THE EYE. 



The history of the eye of Amblyopsis may be divided into four periods. The 

 first period extends from the appearance of the eye till the embryo reaches 4.5 milli- 

 metres in length. This period is characterized by a normal palingenic development 

 except that cell division is retarded. The second period extends from the end of the 

 first period till the larva? are 10 millimetres long. It is characterized by the direct 

 development of the eye from the normal embryonic stage reached in the first period 

 to the highest stage reached by the Amblyopsis eye; its latter half is further charac- 

 terized by the entire obliteration of the lens. The third period extends from the 

 second period to the beginning of old age (10 millimetres to about 80 or 100 millime- 

 tres). It is characterized by a number of changes which, while not improving the 

 eye as an organ of vision, are positive as contrasted with degenerative. There are, 

 however, also distinct degenerative processes taking place during this period. The 

 fourth period is characterized by negative or degenerative processes only. It begins 

 with senescent degeneration * and ends with death at old age. 



* It is questionable whether the changes are senescent in the usual sense of the word. They occur in old 

 age, but at a time when there are no indications of senescence in the fish as a whole. It may be urged 

 that the end product of these degenerative changes should be regarded as the typical structure of the eye of 

 Amblyopsis. In the present paper the structure of the eye at the end of distinctly developmental stages is 

 considered typical. The changes here considered cannot in any way be considered developmental. 



