104 



D. CONCLUSIONS OF GENERAL BIOLOGICAL INTEREST. 



Some late stages of development are omitted by the giving out of de- 

 velopmental 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. 



There being no causes operative or inhibitive either within the fish 

 or in tlie environment that are not also operative or inhibitive in Cholo- 

 gaster 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 accumulation of such degenerative changes 

 as are still notable in the later history of the eye of the adult. 



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

 instead of continuing the i^lan with additional material, completes it out 

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

 foundation of the eye is phylogenic, the stages beyond the foundations are 

 direct. 



