146 BLIND VERTEBRATES AND THEIR EYES. 



6. The steps in degeneration are seen in figure 66, page 176. 



7. The structure of the vestigial eyes differs much in different individuals. 



8. The eye of Chologaster is an eye symmetrically reduced from a larger, normal 

 fish eye. 



9. The retina in Chologaster is the first structure that was simplified. 



10. Later the lens, and especially the vitreous body, degenerated more rapidly 

 than the retina. 



11. The eye of Typhlichthys has degenerated along a different line from that 

 of Awblvopsis, its pigmented epithelium having been most profoundly affected. 



12. The eye muscles have disappeared in Typhlichthys. 



13. Troglichthys shows that the steps in the degeneration of the muscles were in 

 the direction of lengthening their attaching tendons, finally replacing the muscles 

 with strands of connective fibers. 



14. The scleral cartilages have not kept pace in their degeneration with the 

 active structures of the eye. 



15. The lens in the blind species, if present, is, for the most part, a small group 

 of cells without fibers; in Amblyopsis it disappears early. 



16. The proportional degeneration of the layers of the retina is shown in figure 

 67, page 179. 



17. With advancing age the eye of Amblyopsis undergoes a distinct ontogenetic 

 degeneration from the mature structure. 



18. The phyletic degeneration does not follow the reverse order of development. 

 None of the adult degenerate eyes resemble stages of past (phyletic) adult condi- 

 tions. 



19. The degenerate eyes do not owe their structure to a cessation of develop- 

 ment at any past ontogenetic stage, i.e., at any stage passed through in developing 

 a normal eye. 



20. Cessation in development occurs in the reduction of the number of cell 

 generations produced to form the eye and in histogenesis, not in cessation of mor- 

 phogenic processes. 



21. In some cases (Typhlichthys) there is a retardation in the rate of develop- 

 ment, the permanent condition being reached later in life than is usual in fishes. 

 (It is possible that the pigment of the pigment epithelium never comes to develop 

 at all. It is, however, impossible to assert this until the embryos of this species are 

 examined. It is possible that the pigment degenerates before the stages that I have 

 rx;i mined are reached.) 



