102 



B. HISTORY. 



The history of the eye may be divided into four periods: 



(a) The first period extends from the appearance of the eye till the 

 embryo reaches 4.5 mm. in length. This period is characterized by a 

 normal paliiigenic development except that cell division is retarded and 

 there is vei'y little growth. 



(b) The second period extends from the first till the fish is 10 mm. 

 long. It is characterized by the direct development of the eye from the 

 normal embryonic stage reached in the tirst period to the highest stage 

 reached by the Amblyopsis eye. 



(c) The third period extends from the second period to tlie beginning 

 of senescent degeneration, from a length of 10 mm. to about SO or 100 mm. 

 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 also distinct degenerative processes taking place during this 

 period. 



(d) The fourth period begins with the beginning of senescent degen- 

 eration and ends with death. It is characterized by degenerative processes 

 only, which tend to gradually disintegrate and eliminate the eye entirely. 



