NITROGENOUS EXCRETION BY EMBRYOS OF THE

Functional considerations of the placenta of saurian reptiles are limited virtually to assumptions based on known embryonic requirements and structural placental features (Weekes, 1935). It appeared that the nature of embryonic excretion might profitably be investigated in the garter snake whose young are born alive. The extreme solubility of urea, and the fact that the black snake embryo excretes only 20 % of its nitrogenous waste as uric acid (Clark, 1953), suggest the possi-bility that embryos of a placental saurian reptile might pass urea and ammonium salts to the blood stream of the mother; whether some of the soluble nitrogenous excreta are eliminated through the placenta or whether all nitrogenous excreta are retained in the foetus and adnexa, the degree to which the embryo is dependent upon uric acid is of interest. Accordingly, assays of urea, ammonia and uric acid in the developing embryo were undertaken. MATERIALS AND METHODS Urea and ammonia were measured by the technique of Conway (1947), and uric acid was determined colorimetrically according to the method of Brown (1945). Periodically, blood samples were drawn from the mother by severing the tail, and comparisons of urea and ammonia were made with non-pregnant females and males. The measurements of ammonia and urea production by the adult snake were obtained by confining the snake in a glass container, clearing the cloaca by pressure, and washing the snake with 100 ml. water which served as diluent for the excreta. Embryonic tissues were obtained by serial removal from the mother after anaesthetization with Nembutal (Clark, 1937). The tissues for analysis were weighed on a Roller-Smith Balance and a quantity of water was added which would bring the urea or uric acid to a concentration suitable for accurate deter-mination by the techniques employed. The tissues were then homogenized in a Potter homogenizer, and aliquots were taken for assay of ammonia, urea and uric acid. Dry weights were obtained by treatment in a vacuum oven at 300 C. at approxi-mately 50 mm. pressure to constant weight. Protein assays were made according • This investigation was supported in part by a research grant (G—3827) from the Nationa