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In the Anablepidae, a family of viviparous Cyprinodont fishes, the mature eggs are fertilized in the ovarian follicles. The embryos are retained in the modified ovarian follicles until birth and are born when about 44 mm. in length. The embryos develop expanded yolk sacs which reach a maximal diameter of about 10. mm. The vitelline veins on the surface of the yolk sac develop rows of bead-like swellings, yolk sac bulbs, which serve to facilitate absorption of materials from the fluid of the follicle cavity. There is an extensive system of follicular villi upon the internal lining of the follicular capsule, the apparent function of which is to increase the vascular surface of the internal lining of the follicular capsule and to facilitate the interchange of materials between the blood of parent and fluid of the follicular cavity. The follicular villi and the yolk sac bulbs develop at the same time. These two adaptations for viviparity are apparently unique in this family of fishes.

Viviparity in the teleost fishes is most highly developed in the family Embiotocidae. In Cymatogaster aggregatus, a member of this family, the males are sexually mature at birth. Females born in June and July are inseminated soon after birth and carry the spermatozoa in the ovarian cavity until December when eggs are fertilized. Embryos are retained in the ovarian cavity till the following spring when they are born. The cells of the epithelium lining the ovarian cavity develop internal fluid reservoirs which reach their maximal stage in early gestation when there is an extensive sloughing of epithelial cells. The epithelium returns to its original condition in late gestation and later the cells undergo multiplication. The stroma becomes swollen and contains a large amount of fluid during early gestation and shrinks as the embryos become larger. Many of the eggs, failing to reach maturity at the time of fertilization, degenerate. Degeneration also takes place in numerous smaller ovocytes during gestation but some of them survive to grow during the next season. The cyclical changes in the ovarian tissue of Cymatogaster aggregatus are parallel to those which occur in the ovaries of the Goodeidae and the Jenynsiidae, two other families in which viviparity has been highly developed.

The Goodeidae are a family of cyprinodonts containing about nine genera and twenty-six species. They are confined exclusively to the Mexican Plateau and to streams in the immediate vicinity. All are viviparous. Fertilization is internal. The eggs are extremely small and contain very little yolk. The embryos are retained in the ovarian follicles until the yolk is practically absorbed, in the meantime developing unique absorptive organs in the form of extensive ribbon-shaped proctodaeal processes. The embryos are evacuated into the intra-ovarian cavity where they are retained for several weeks, during which time the proctodaeal processes become extended. The ovary becomes a nutritive organ and produces secretions which are discharged into the intra-ovarian cavity and absorbed by the embryo through their unique processes. The processes differ in form, number, and histological structure, but are specific in their peculiarities for each species. These specific differences, together with marked differences in the ovary, will furnish a basis for a re-classification of the genera and species of the family. A comparison in made between this family and the other ovo-viviparous and viviparous cyprinodonts, on the basis of which it is concluded that the Goodeidae have arisen separately from an ovo-viviparous ancestor, which in turn was derived from an ovo-viviparous type. A comparison is also made between the peculiarities of reproduction in this family and those of other viviparous teleosts.

Parabrotula dentiens is a viviparous deep sea Brotulid fish. The embryos at the 6 mm. stage are held within the ovarian cavity as are the embryos of Zoarces, the Embiotocidae, the Goodeidae, the Jenynsiidae and the Anablepidae in the later stages. The embryos of Parabrotula dentiens are equipped with external processes which are attached around the anus and to the ventral wall of the embryo for a short distance in front of the anus. The processes have a spongy connective tissue stroma and are covered with a syncytial layer. It is assumed that the processes augment the body surface of the embryos and have a nutrition-absorbing and an oxygen-absorbing function.

The embryos of twenty-one species of the family Goodeidae have extensive rectal processes (trophotaeniae) which serve as absorptive organs, by means of which the embryos while they are retained in the ovarian cavity absorb substances dissolved in the ovarian fluid. Embryos of three species have not been available for study. No trophotaeniae are present in one species Ataeniobius toweri. There are three general types of trophotaeniae, rosette, sheathed and unsheathed. The structure of the trophotaeniae is sufficiently constant in each species to be used in species determination.