174 BLIND VERTEBRATES AND THEIR EYES. 



ii. For a summary of the origin, development, and degeneration of the eye and 

 its parts see table, page 164. 



12. There is no constant ratio between the extent and degree of ontogenetic 

 and phylogenetic degeneration. The observed rate of ontogenetic degeneration 

 is not necessarily proportionate to the rate of phylogenetic degeneration inferred 

 from the degree of degeneration of the eye at its optimum. 



13. If Troglichthys indicates one of the steps through which the eye of Ambly- 

 opsis will pass to 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. 



14. Some late stages of development are omitted by the giving out of develop- 

 mental processes. Some of the processes giving out are cell division, resulting in 

 the minuteness of the eye and the histogenic changes which differentiate the cones 

 and the outer reticular layer. 



15. There being no causes operative or inhibitive either within the fish or in 

 the environment that are not also operative or inhibitive in Chologaster agassizii, 

 which lives in caves and develops well-formed eyes, it is evident that the causes 

 controlling the development are hereditarily established in the egg by an accumu- 

 lation of such degenerative changes as are still notable in the later history of the 

 eye of the adult. 



16. The foundations of the eye are normally laid, but the superstructure, 

 instead of continuing the plan with additional material, completes it out of the 

 material provided for the foundations. The development of the foundation of the 

 eye is phylogenetic, the stages beyond the foundations are direct. 



