218 E. M. WiDDOWsoN AND R. A. McCance 



concentration of potassium does not fall in the same way, and 

 the concentration of calcium rises. This calcium appears to 

 be held in solution by citric acid (Economou-Mavrou and 

 McCance, 1958). 



The fluid at 45 days has been found to have a pH between 

 5-5 and 6, and a titratable acidity of about 10 m-equiv./litre. 

 The fluid contains ammonia, and ammonia appears to ac- 

 count for about 25 per cent of the titratable acid plus ammonia 

 found in it. The concentration of phosphates is always small, 

 and the acidity is almost entirely due to carbonic acid. The 



pH rises quickly if the fluid is shaken or even if it is left in a 

 tube exposed to the air, and it was found necessary to collect 

 and analyse the fluid out of contact with air. Lutwak-Mann 

 and Laser (1954) found no "bicarbonate" in pig's allantoic 

 fluid at 20 days' gestation, but there is no doubt about the 

 presence of carbonic acid at 45 days. 



Further investigation has confirmed the fact, first noted by 

 Lutwak-Mann (1955), that the chorioallantoic membrane 

 contains carbonic anhydrase. At 45 days the allantoic fluid 

 itself also had some carbonic anhydrase activity. On the 

 basis of material from three pregnant pigs the activities of 

 carbonic anhydrase may be given as foetal kidney + + +, 

 chorioallantoic membrane ++, allantoic fluid +• It is 



