496 



SCIENCE. 



[N. S. Vol. XI. No. 274. 



of the ej-e, and various projectious and 

 angles in their endeavor to accommodate 

 themselves to the small structure which 

 they cover. 



This is in striking contrast to the con- 

 ditions in Typhlotriton, where but a slight 

 nodule of cartilage remains and this is very 

 frequently absent in the adult, while in the 

 larva of the same species a cartilaginous 

 band extends almost ai'ound the equator of 

 the eye. The diiferent effect of degenera- 

 tion in the Amblyopsid* and the salamand- 

 ers could not be more forcibly illustrated 

 than by the scleral cartilages.* 



D. The eye as a whole and its different 

 parts may now be considered. 



1. The dioptric apparatus. 



The steps in degeneration of the eye in 

 general are indicated in the accompanying 

 figure. 



The most highly developed eye is that of 

 Chologader papilliferxis. The parts of this 

 ej'e are well proiDortioned, but the eye as a 

 whole is small, measuring less than oue mil- 

 limeter in a specimen 55 mm. long. The 

 proportions of this eye are symmetrically re- 

 duced if it has been derived from a fish eye 

 of the average size, but the retina is much 

 simpler than in such related pelagic species 

 as Zygonectes. The simplifications in the 

 retina have taken place between the outer 

 nuclear and the ganglionic layers. The 

 pigment layer has not been materially af- 

 fected. These facts are exactly opposed to 

 the supposition of Kohl that the retina and 

 the optic nerve are the last to be affected 

 and that the vitreous bodj'^ and the lens 

 cease to develop early. In Chologader papil- 

 liferus the latter parts are normal, while the 

 retina is simplified. That the retina is af- 

 fected first is proved beyond cavil by Cholo- 

 gader eornufus. The vitreous body and the 

 lens are here larger than 'papilliferus, but the 



* The presence of a cartilaginous band in the young 

 is, possibly, a larval character, and its absence in the 

 adult has, in that case, no bearing on degeneration. 



retina is very greatly simplified. Cornutus, 

 it must be borne in mind, lives in the open. 

 The eye of the cave species Chologader agas- 

 sizil ditfers from that of papilliferus largely 

 in size. There is little difference in the 

 retinas except the pigmented laj'er, which 

 is about 26 per cent, thinner in agassizii than 

 in papilliferus. 



There is a big gap between the lowest eye 

 of Chologaster and the highest eye of the 

 blind members of the AmblyopsidiB. The 

 lens in the latter has lost its fibrous nature 

 and is merely an ill-defined minute clump 

 of cells scarcely distinguishable in the ma- 

 jority of cases. The vitreous body of the 

 latter species is gone with perhaps a trace 

 still remaining in Typhlichthys. With the 

 loss of the lens and the vitreous body the 

 eye collapsed so that the ganglionic laj'er 

 formerly lining the vitreous cavity has been 

 brought together in the center of the eye. 



The layers of the retina in Typhlichthys 

 are so well developed that could the vitreous 

 body and lens be added to this eye it would 

 stand on a higher plane than that of Cholo- 

 gaster cornutus, exclusive of the cones and 

 pigmented layer. It is generally true that 

 at first the thickness of the layers of the 

 retina is increased as the result of the re- 

 duction of the lens and vitreous body and 

 the consequent crowding of the cells of the 

 retina whose reduction in number does not 

 keep pace with the reduction in the dioptric 

 apparatus in total darkness. 



If we bear in mind that no two of the 

 eyes represented here are members of a 

 phyletic series, we may be permitted to state 

 that from an ej'e like that of cornutus, 

 but possessing scleral cartilages, both the 

 ej'es of Amblyopsis, and Troglichthys have 

 been derived and that the eye of Amblyopsis 

 represents one of the stages through which 

 the eye of Troglichthys passed. The eye of 

 Amhlyopsis is the eye of C. cornutus minus a 

 vitreous body with the pupil closed and with 

 a minute lens or none at a)l. The nuclear 



