An Accessory Respiratory Device in Embryos of the Embiotocid Fish, Cymatogaster aggregata, during Gestation

sively from the fluid of the ovarian cavity of the mother. Secretions from the cells lining the maternal ovarian cavity, and also fluid from the swollen ovary, contribute to the nutritive supply. The fluid is fortified also (Turner, 1938) by epithelial cells desquamated from the internal lining of the ovarian cavity and also by the remains of dead and partially absorbed embryos. The ovarian liquor enters the alimentary canals of the embryos by way of the mouths or opercular openings, and nutritive materials are absorbed from it by the intestine which is modified for the purpose. Nitrogenous wastes and partially dehydrated residues from the alimentary canals of the embryos are discharged into the ovarian cavity in which the embryos are retained and live. Presumably the toxic compounds are absorbed through the vascular ovarian tissues of the mother and are eliminated by the maternal excretory organs. The respiratory requirements of the early embryos are not great and apparently are met by the vascular portal system covering the body wall of the pericardial and peritoneal cavities. These portal systems are bathed by the maternal ovarian fluid. During later stages of development, there is an opportunity for exchange of oxygen and carbon dioxide between the gills of the embryo and the ovarian fluid as the fluid is drawn into the alimentary canal through the pharynx. Structures adapted for respiratory exchange are developed in the older embryos and appear as spatulate vascular expansions of the tissue at the margins of the dorsal and anal fins. These structures are lost near the close of the gestation period. An undescribed structural feature which facilitates respiration has been discovered in embryos ready for birth and is described in the following paragraph. At the close of the period of gestation the embryos are closely packed within the ovarian cavity of the mother and are in contact with the ovarian walls or the ovarian ovigerous lamellae. In approximately one-half of the embryos, the opercular opening on one side has been invaded by a fold of tissue from the wall or one of the septa of the ovary to a depth of about 2 mm,