1GG BLIND VERTEBRATES AND THETR EYES. 



THE FUTURE OF THE EYE. 



There can be no doubt that the phylogenetic fate of the eye, exclusive of con- 

 nective tissue, sheaths, sclera, etc., is total disappearance. The most degenerate 

 ontogenetic eye indicates as much. There are no relatives of Amblyopsis that have 

 reached this condition, but Troglichthys has an eye distinctly more degenerate than 

 that of Amblyopsis. It may offer a clew as to whether any of the ontogenetically 

 degenerate eyes, such as are found in old specimens of Amblyopsis, are prophetic 

 of the condition through which the eye will pass in its route to the vanishing point. 

 The most highly developed eye found in any specimens of Troglichthys (plate 10, 

 fig. h) is comparable in a general way with the eyes of the old of Amblyopsis. The 

 pigmented epithelium is larger than the requirements of the eye in both cases, 

 and the scleral cartilages are disproportionately developed in both cases. The 

 ganglionic cells extending through the center of the eye of the younger Amblyopsis 

 are absent in both cases. Only 3 cells have been found in this region in all the eyes 

 of Troglichthys examined. When we attempt a closer comparison, our efforts fail. 



We may conclude that if Troglichthys indicates one of the steps through which 

 the eye of Amblyopsis will pass to its annihilation, the degenerative phases seen in 

 the oldest specimens of Amblyopsis indicate only in a general way the phylogenetic 

 path over which the eye will pass in the future. 



RETARDATION AND CUTTING OFF OF LATE STAGES OF THE DEVELOPMENT 



OF THE EYE. 



In my first paper on the Eyes of Blind Vertebrates (Roux' Arch. viii. p. 596, 

 1899) I said: 



Cessation of development takes place only in so far as the number of cells are concerned. 

 The number of cell generations produced being continually smaller results in an organ as a con- 

 sequence also smaller. In this sense we have a cessation of development (cell division, not 

 morphogenic development) in ever earlier stages. That there is an actual retardation of devel- 

 opment is evident from Amblyopsis and Typhlkhthys in which the eye has not reached its final 

 form when the fish are 35 mm. long. 



I am convinced now that this statement did not go far enough. There is, indeed, 

 a gradual retardation in all processes of development which frequently terminates in 

 a complete arrest of development before the final stages of normal eyes are reached. 

 This is especially true of the lens. In discussing the changes it will be best to keep 

 separate the three groups of processes concerned in development. 



The proof of the limiting of the number of cell divisions mentioned has been 

 brought out in the chapters on the development. It has also been seen that the rate 

 of division is very much retarded. In the retina it stops altogether at the time the 

 fish has reached a length of 5 to 7 mm., and very rarely more than two dividing 

 cells are found in any eye. In its first stages the eye is thus about equal in size to 

 the adult eye. Cell division stops earlier in the lens, where no new cells are formed 

 after it is cut off from the skin. The lens is at this time relatively as well developed 

 as the retina. In both the retina and the lens cell division ceases in late stages, and 

 the total number of cell generations is very much limited. The lens is looked upon 

 as phylogenetically a new structure, and we have, by the stopping of its later stages 

 of cell division, a step in the elimination of a phylogenetically new structure. This 

 is, however, of no consequence because it is not differential, for the retina, a phylo- 

 genetically older structure, suffers a similar stoppage. There is no evidence, then, 



