MARCli 30, 1900.] 



SCIENCE. 



499 



layer is simplified, the nerve is measurably 

 thinner. In Typhlichthys the nerve can be 

 traced to the brain in specimens 40 mm. 

 long, i. e. , in specimens which are evidently 

 adult. In Amblyopsis the nerve can be fol- 

 lowed to the brain in specimens 25 mm. 

 long, but in the adult I have never been able 

 to follow it to the brain. In Trogliehthys it 

 has become so intangible that I have not 

 been able to trace it far beyond the eye. 



We thus see that the simplification or 

 reduction in the eye is not a horizontal 

 process. The purely supporting structures 

 like the scleral cartilages have been re- 

 tained out of all proportion to the rest of 

 the eye. The pigment layer has been both 

 quantitatively and qualitatively differently 

 affected in different species. There was 

 primarily an increase in the thickness of 

 this layer and later a tendency to total loss 

 of pigment. The degeneration has been 

 more uniformly progressive in all the layers 

 within the pigment layer. The only possi- 

 ble exception being the inner recticular 

 layer which probably owes its retention 

 more to its supporting than to its nervous 

 elements. Another exception is found in 

 the cones, but their degree of development 

 is evidently associated with the degree of 

 development of the pigmented layer. As 

 long as the cones are developed the pig- 

 mented layer is well developed or vice 

 ve^'sa. 



We find in general then that the reduction 

 in size from the normal fiSh eye went hand 

 in hand with the simplification of the retina. 

 There was at first chiefly a reduction in the 

 number of many times duplicated parts. 

 Even after the condition in Chologaster papil- 

 lifenis was reached, the degeneration in the 

 histological condition of the elements did 

 not keep pace with the reduction in number 

 (vide, the eye of cornutus) . The dioptric 

 apparatus disappeared rather suddenly and 

 the eye as a consequence, collapsed with 

 equal suddenness in those members which, 



long ago, took up their abode in total dark- 

 ness. The eye not only collapsed, but the 

 number of elements decreased very much. 

 The reduction was in the horizontally 

 repeated elements. The vertical complex- 

 ity, on which the function of the retina 

 really depends, was not greatly modified at 

 first. 



In those species which took up their 

 abode in total darkness the degeneration 

 in the dioptric apparatus was out of pro- 

 portion to the degeneration of the retina, 

 while in those remaining above gi-ound the 

 retinal structures degenerated out of pro- 

 portion to the changes in the dioptric ap- 

 paratus, which, according to this view de- 

 generates only under conditions of total 

 disuse or total darkness which would neces- 

 sitate total disuse. This view is upheld by 

 the conditions found in Typholgobius, as 

 Ritter's drawings and my own preparations 

 show. In Typholgobius the eye is functional 

 in the young and remains a light perceiv- 

 ing organ throughout life. The fish live 

 under rocks between tide water (Eigen- 

 mann, 90). We have here an eye in a con- 

 dition of partial use and the lens is not 

 affected. The retina has, on the other 

 hand, been horizontally reduced much more 

 than in the Amblyopsidse, so that should the 

 lens disappear, and Ritter found one speci- 

 men in which it was gone, the type of eye 

 found in Trogliehthys would be reached with- 

 out passing through a stage found in Ambly- 

 opsis, it would be simply a horizontal con- 

 tracting of the retina, not a collapsing of 

 the entire eye. 



The question may with propriety be asked 

 here, do these most degenerate eyes ap- 

 proach the condition of the pineal eye ? It 

 must be answered negatively.* 



*The degree of degeneration reached in the eye of 

 TrogHMhys which began to degenerate comparatively 

 recently would lead one to expect the pineal eye to be 

 much more degenerate than it is actually found to be 

 in the lizards unless its functions were something 

 aside from light perception. 



