532 THE PHYSIOLOGY OF REPRODUCTION 



fact that in pregnancj' the mother retains nitrogen in amounts quite 

 out of proportion to its increase in hody weight. In what form this 

 nitrogen is retained is not yet clear. The nitrogen retention does 

 not proceed evenly throughout pregnancy, but is most marked in the 

 second half of pregnancy, so that we have a confirmation of the 

 paradox observed by Bar in dogs that the nitrogen retention is 

 greatest when the requirements of the foetus are highest. The 

 increased retention is due to diminished excretion in the urine. It 

 follows, therefore, that in a normal pregnancy the maternal organism 

 improves its capacity for retaining nitrogenous material, and as we 

 shall see presently, also inorganic materials. The observations of 

 Hoffstrom have been confirmed by Wilson,^ who also found a retention 

 of nitrogen in excess of the requirements of both the foetus and the 

 mammary glands and genitalia of the mother. The store of nitro- 

 genous material is apparently a reserve which is drawn upon and 

 exhausted during the puerperium and lactation. It has been shown 

 recently^ that the retention of nitrogen is associated with the 

 functional activity of the lymphoid tissue. If the latter is atrophied 

 the organism loses nitrogen and the animal emaciates. If the 

 lymphoid tissue is very active, there is a greatly increased nitrogen 

 retention. It is possible that the increased nitrogen retention in 

 pregnancy is associated with the leucocytosis of pregnancy which 

 has been observed frequently (see below, p. 556). This suggestion 

 finds confirmation in the observation referred to above, that a gain 

 in weight during pregnancy of the maternal organism can be ensured 

 by an abundant supply in the diet of the water-soluble vitamin 

 which stimulates the functional activity of lymphoid tissue. 



(d.) The Excretion of Nitrogen during Pregnancy and its Distribution 

 in the Urine. — -It has just been stated that the excretion of nitrogen 

 diminishes as pregnancy progresses. The excretion by the intestine 

 remained constant in Hoffstrom's case. Bar's observations on dogs 

 also showed that the faecal nitrogen was, if anything, diminished. 

 This indicates that the absorption and utilisation of food by the 

 intestine remains good during a normal pregnancy. The excretion 

 of nitrogen by the urine is diminished, and it is by virtue of this 

 diminution that the maternal organism retains increasing amounts 

 of nitrogen. This is due to a diminished excretion of urea in the 

 urine, as both Bar and Murlin showed. The result is an alteration 

 of the nitrogen distribution in the urine : the percentage of nitrogen 

 excreted in the form of urea is diminished, the percentage excreted 

 in the form of ammonia, the so-called " ammonia index '' or " ammonia 



1 Wilson (K. M.), " Nitrogenous Metabolism in Pregnancy," Johns Hopkins 

 Hasp. Bull., vol. xxvii., 1916. 



^ Cramer, Drew, and Mottram, loc. eit. 



