The Composition and Origin of the Foetal Fluids of the Pig

MANY of the unicellular organisms have a greater concentration of osmotically active material within their body fluids than there is in the medium surrounding them, and the higher animals have glands like the kidney and sweat glands which are able to elaborate fluids with much lower concentrations of sodium and chloride than the body-fluids from which they were derived. The concentrations of sodium and chloride in hypotonic urine and sweat are much lower than those in the serum, and consequently the total osmolar concentrations are also lower, but the urea is characteristically higher. There is no proof that the process is one involving the secretion of water; and, indeed, the kidney is now thought to produce a hypotonic urine by the active reabsorption of sodium from the distal tubule, the walls of which are impermeable to water unless there is posterior pituitary hormone in the circulation. The foetal fluids and membranes deserve more consideration in this connexion than they have hitherto received. Jacque (1903-4, 1905-6) pointed out that the allantoic fluid of the sheep was hypotonic and propounded the theory that this fluid was largely foetal urine which he also demonstrated to be hypotonic. This idea has been brought up to date by McDougall (1949) and by Alexander, Nixon, Widdas, & Wohlzogen (1955 a, b). Paton, Watson, & Kerr (1908-9) discussed the whole question of the origin of foetal fluids—human and otherwise—in the light of the knowledge which existed at that time. There is no doubt, as they pointed out, that these fluids are of foetal origin, but the progress of knowledge as to their nature and origin has always been obscured by (i) the great variability in their volume and composition even between littermates and certainly with the progress of gestation, (ii) obvious species differences, (iii) a tendency to compare their composition with foetal urine, rather than with foetal or maternal serum, (iv) a failure to appreciate the importance of their total osmolar concentration, and (v) neglecting to search for general principles and generalizations. Davies & Routh (1957) have recently made out a strong case for regarding the allantoic fluid of the rabbit as the product of the foetal mesonephros. Their essential findings have been confirmed (Dickerson & McCance, 1957) and the