1. A short account is given of the breeding habits of Heterandria formosa in an aquarium.
2. The ovary and the mature ovum are briefly described, the noteworthy feature of the ripe ovum being the small quantity of yolk.
3. Degeneration of ova is found to be a common occurrence in unfertilized females, and to a less extent in those full of developing embryos.
4. The method for ensuring the fertilization of the egg within the follicle is portrayed. Over the area where the spermatozoa have entered the cells of the ovarian epithelium and those of the follicle form a solid plug which eventually disrupts to enable the fully developed embryo to escape into the cavity of the ovary.
5. It is characteristic of early development that the egg is encircled by a unilaminar ectoderm before there is any visible differentiation into endoderm and mesoderm.
6. Owing to the scarcity of yolk only a few periblast cells arise and no syncytial layer is formed.
7. The primitive-germ cells are visible at an early stage within the apparently undifferentiated mesendoderm cells.
8. A striking feature is the large size of the pericardium and its growth upwards as a pericardium hood which completely surrounds the head region of the embryo. Over the walls runs a network of blood-vessels from which the maternal capillaries become eventually separated by only an attenuated layer of protoplasm. Both respiration and nutrition are effected through the follicle.
9. A remarkable specialization is the development of a urinary bladder which expands into a thin-walled vesicle of enormous dimensions, finally occupying almost the entire area formerly filled by the pericardium and the yolk-granules.
10. The development of Heterandria is compared with that of other viviparous fishes.
11. The significance of the unusual features in early and late development is discussed and some comparisons are made with the conditions in higher vertebrates.