"(_a_) The first extends from the appearance of the eye till the embryo is 4-5 mm. long. This period is characterized by a normal palingenic development, except that the cell division is retarded and there is very little growth. [Illustration: FIG. 164.--Agonoid-fish, _Pallasina barbata_ (Steindachner). Port Mulgrave, Alaska.] "(_b_) The second period extends till the fish is 10 mm. long. It is characterized by the direct development of the eye from the normal embryonic stage reached in the first period to the highest stage reached by the _Amblyopsis_ eye. [Illustration: FIG. 165.--Blindfish of the Mammoth Cave, _Amblyopsis spelæus_ (De Kay). Mammoth Cave, Kentucky.] "(_c_) The third, from 10 mm. to about 80 or 100 mm. It is characterized by a number of changes which are positive as contrasted with degenerative. There are also distinct degenerative processes taking place during this period. "(_d_) The fourth, 80-100 mm. to death. It is characterized by degenerative processes only. "The eye of _Amblyopsis_ appears at the same stage of growth as in normal fishes developing normal eyes. The eye grows but little after its appearance. "All the developmental processes are retarded and some of them give out prematurely. The most important, if the last, is the cell division and the accompanying growth that provide material for the eye. "The lens appears at the normal time and in the normal way, but its cells never divide and never lose their embryonic character. "The lens is first to show degenerative steps and disappears entirely before the fish is 10 mm. long. [Illustration: FIG. 166.--Blind Brotula, _Lucifuga subterranea_ (Poey), showing viviparous habit. Joignan Cave, Pinar del Rio, Cuba. Photographed by Dr. Eigenmann.] "The optic nerve appears shortly before the fish reaches 5 mm. It does not increase in size with the growth of the fish and disappears in old age. "The scleral cartilages appear when the fish is 10 mm. long; they grow very slowly, possibly till old age. "There is no constant ratio between the extent and degree of ontogenic and phylogenic degeneration. "The eye is approaching the vanishing point through the route indicated by the eye of _Troglichthys rosæ_. "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 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 plan 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."