﻿180 BLIND VERTEBRATES AND THEIR EYES. 



ONTOGENETIC DEGENERATION. 



The simplification of the eye in cormUus has been mentioned in the foregoing 

 paragraphs. It may be recalled that the nuclear layers are thinner in the old than 

 in the young. There is here not so much an elimination or destruction of clement 

 as a simplification of the arrangements of parts, comparatively few being present 

 to start with. 



The steps in ontogenetic degeneration can not be given with any degree of finality 

 for Amhlyopsis on account of the great variability of the eye in the adult. While 

 the eyes of the very old have unquestionably degenerated, there is no means of 

 determining what the exact condition of a given eye was at its prime. In the largest 

 individual examined the eye was on one side a mere jumble of scarcely distinguish- 

 able cells, the pigment cells and scleral cartilages being the only things that would 

 permit its recognition as an eye. On the other side the degi-ee of development 

 was better. 



The fact that the eyes are undergoing ontogenetic degeneration may be taken, 

 as suggested by Kohl, that these eyes have not yet reached a condition of equilibrium 

 with their environment or the demands made upon them by use. Furthermore, 

 the end result of the ontogenetic degeneration is a type of structure below anything 

 found in the phlyogcny of the vertebrate eye. It is not so much a reduction of the 

 individual parts as it is a wiping out of all parts. 



PLAN AND PROCESS OF PHYLETIC DEGENERATION IN THE AM BLYOPSl D>^. 



Does degeneration follow the reverse order of development or does it follow 

 new lines, and if so, what determines these lines ? Since the ontogenetic development 

 of the eye is supposed to follow in general lines its phyletic development, the above 

 question resolves itself into whether or not the eye is arrested at a certain stage of 

 its morphogenic development, and whether this causes certain organs to be cut off 

 from the development altogether^ In this sense the question has been answered 

 in the affirmative by Kohl. Ritter, while unable to come to a definite conclusion, 

 notes the fact that in one individual of Typhlogobius the lens which is phyletically 

 a new structure had disappeared. This lens had probably been removed as the 

 result of degeneration rather than through the lack of development. Kohl supposes 

 that in animals placed in a condition where light was shut off more or less, every 

 succeeding generation developed its eye less. Total absence of light must finally 

 prevent the entire anlagc of the eye. Time has not been long enough to accomplish 

 this in any vertebrate. Phyletic degeneration is looked upon as the result of a long 

 series of " H mmungen " which in successive generations appeared in ever earlier 

 time of ontogenetic development in always lower stages of the development of the 

 individual eyes. The eye develops after the vertebrate type. At certain stages 

 the rate of progress is diminished and in most cases finally completely ceases. A 

 retardation has developed which after a shorter or longer period ends in the cessa- 

 tion of all development. The first appearance of the retardation falls in a time 

 of embryonic or post-embryonic development that in the phylogeny corresponds 

 to the moment when the lack of light became operative. The period in ontogeny 

 which lies between the first disturbance in development and its cessation corre- 

 sponds to the phyletic time during which the development of the eye is checked at 

 a continually lower stage of development. The point of cessation in ontogeny cor- 

 responds to the time when the eye reached its equilibrium. If in ontogeny there 



