Development of Acid-Base Control 217 



is more than the amount excreted by the adults in this series. 

 In so far as citric acid may be regarded as a product of the 

 metaboHsm of the kidney it cannot be classed as a surplus 

 anion although it contributes to the titratable acidity. 



It is well known that infants on cows' milk mixtures have a 

 higher concentration of inorganic phosphorus in their serum 

 than breastfed infants; they excrete phosphates by the 

 seventh day of life, and the phosphate-organic acid relation- 

 ship is of the adult pattern, as it is also in the urine of infants 

 eight months to one year of age. 



Foetal Life 



In the uterus the acid-base balance of the whole conceptus 

 is regulated ultimately by the mother's lungs and kidneys, 

 but the foetal kidneys, membranes and placenta act as inter- 

 mediaries. 



Urine has been taken from the bladders of five human 

 foetuses aged 10-20 weeks. It has always been found to be 

 hypotonic, due mainly to very low concentrations of sodium 

 and chloride. It appears to resemble the urine formed in 

 utero and passed at term which has been better investigated 

 and described elsewhere (McCance and Widdowson, 1953; 

 Hanon, Coquoin-Carnot and Pignard, 1955, 1957). 



The pig has a gestation period of about 120 days. Between 

 the 20th and 60th day there is a rapid expansion in the volume 

 of allantoic fluid. The sac containing the fluid has free con- 

 nexion with the kidney through the urachus and bladder. 

 Its membranes also participate in exchanges with the mother. 

 Table I shows the composition of the fluid at 20 days, 45 days 

 and 60 days. At 45 days both mesonephros and metanephros 

 are functional, but the former is becoming less so. The 

 volume of fluid in the sac is very variable (Wislocki, 1935), but 

 it far exceeds the weight of the foetus. The osmolar concen- 

 tration falls greatly so that from 45 days it is only one-half 

 or one-third that of foetal serum (McCance and Dickerson, 

 1957). This fall in osmolar concentration is due largely to 

 a fall in the concentration of sodium and chloride. The 



