160 BLIND VERTEBRATES AND THEIR EYES. 



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 mm. in length (plate 10, 

 fig. b). The optic nerve is evident within the eye in older stages up to about 

 100 mm.; in the very oldest ones it could not be found. In individuals much 

 more than 25 mm. 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 fibers are never 

 medullated, and so far I have not been able to give them a differential stain. 



HISTORY OF THE DEVELOPMENT, MATURITY, AND DEGENERATION OF THE EYE. 



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



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 palingenetic devel- 

 opment except that cell division is retarded and there is very little growth. 



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 first period to the highest stage reached by the Amblyopsis 

 eye ; its latter half is further characterized by the entire obliteration of the lens. 



The third period extends from the second period to the beginning of senescent 

 degeneration, from a length of 10 mm. to about 80 or 100 mm. It is character- 

 ized 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 degen- 

 erative processes taking place during this period. 



The fourth period begins with the beginning of senescent degeneration and 

 ends with death. It is characterized by degenerative processes only, which tend 

 to gradually disintegrate and eliminate the eye entirely. It is questionable 

 whether these changes should be called senescent. It may be urged that they are 

 the result of disuse in the individual, or that the end product of these degenerative 

 changes is the typical structure of the eye of Amblyopsis. 



First Period. — During the first period the eye arises as a solid outgrowth 

 from the solid central nervous system when the embryo is about 1.5 mm. long. 

 The outgrowth increases rapidly in size during the next 0.5 mm. of growth in 

 length. The solid lateral outgrowth is bent back along the side of the brain, and 

 its connection with the brain becomes constricted into the optic stalk. A cavity 

 approximately central arises in the optic lobe at the same time that a cavity ap- 

 pears in the central nervous system, which occurs when the embryo is about 2 mm. 

 in length. The two layers of the optic vesicle formed by the appearance of the 

 cavity are of about equal thickness. A little later the secondary optic vesicle is 

 formed by the thickening of the skin over the eye and the consequent cupping of 

 the distal face of the eye. The process reaches its culmination when the embryo 

 has a length of 4.4 mm. The lens is still connected with the skin, and the two 

 layers of the secondary vesicle have become very different, the proximal one being 

 one-layered, the distal one several -layered. The details of the changes of this 

 period have been given in the preceding pages. 



At any time up to this length the eye might, as far as its structure is concerned, 

 give rise to a perfect eye in the adult. The eye so far follows phylogenetic paths 

 with the reservation that no adult ancestor is supposed to have had eyes like these 

 embryonic stages. 



