The composition of the foetal fluids of the rabbit at different stages of gestation has been described with reference to nitrogenous materials, electrolytes, total reducing substance, and fructose.

The composition of the amniotic and exocoelomic fluids resembles that of the foetal biood-serum except for the low total protein of the former. These findings are consistent with the origin of these fluids as a dialysate of the foetal blood. The allantoic fluid is characteristically opalescent largely due to the presence of colloidal fat. Its composition also differs from that of the foregoing fluids. Thus the total protein is lower while the level of urea nitrogen and creatinine is higher. Sodium, chlorides, and inorganic phosphates are lower; calcium and potassium are higher. Total reducing substances are about the same in all three fluids when allowance is made for the higher non-fermentable reducing substance of the allantoic fluid: all are lower than the foetal blood-levels. The fructose content is low and about the same in all fluids.

The presence of allantoic fluid from about the 13 th to the 24th day of gestation coincides with the period of function of the foetal mesonephros as judged by embryological and histological criteria. The structure of the nephron of the mesonephros is briefly described and the presence of alkaline phosphatase, fat, and periodic acid Schiff-reacting substances in the proximal tubules is illustrated. The evidence is consistent with the origin of the allantoic fluid as wholly or in part a product of the foetal mesonephros.

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