Adaptations of Fishes 



221 



According to his observations 



" The history of the eye of Amblyopsis spelceus may be divided 

 into four periods: 



Fig. 16.3. — Sleek Sculpin, Psycltrolutes paradoxus (Giinther). Puget Sound. 



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

 the embryo is 4.5 mm. long. This period is characterized by 



Fig. 164. — Agonoid-fish, Pa??a.sma barbata (Steindachner). Port Mulgrave, Alaska. 



a normal palingenic development, except that the cell division 

 is retarded and there is very little growth. 



" (b) The second period extends till the fish is 10 mm. long. 

 It is characterized by the direct development of the eye from 



Fig. 16.5. — Blind-fish of the Mammoth Cave, Amblyopsis spelwus (DeKay). 

 Mammoth Cave,, Kentucky. 



the normal embryonic stage reached in the first period to the 

 highest stage reached by the Amblyopsis eye. 



" (c) The third, from 10 mm. to about 80 or 100 mm. It is 

 characterized by a number of changes which are positive as con- 



