76 PROTEIN REQUIREMENTS OF CATTLE: MITCHELL 



compositing protein requirements from the factors studied separately in 

 this report. For example, the occurrence of pregnancy in a growing 

 heifer may retard growth somewhat, while lactation is known to do so. 

 Hence, the protein requirement of a pregnant lactating heifer would 

 presumably be somewhat less than that indicated by the sum of the 

 separate requirements. On the other hand, the protein requirements for 

 pregnancy considered above have not included any estimates for the 

 hypertrophy of the uterus, no basis for such estimates being available. 

 This error, therefore, would tend to compensate for any depression of 

 growth caused by pregnancy. The occasional occurrence of twin births 

 in cattle (99) is another special problem not touched upon by this report. 



The effect of pregnancy upon the nitrogen metabolism of the mother 

 has been the subject of a number of researches, and the results obtained 

 have lent support to two theories. One theory pictures pregnancy as a 

 distinct sacrifice of the mother to the perpetuation of the species. This 

 theory is based upon a number of studies on dogs and rabbits, which have 

 been ably reviewed by Murlin(^9^^ whose own data upon dogs are taken 

 to support the theory. According to this theory, the total nitrogen stored 

 by the maternal organism during pregnancy is not sufficient to form the 

 fetus and its membranes; thus, the maternal tissues themselves are sac- 

 rificed. As a result, during the first half of the period of pregnancy a 

 negative nitrogen balance persists, especially during the third and fourth 

 week in the dog ; not until the last half of pregnancy is a positive balance 

 attained. It may be said, however, that not all of the observations upon 

 dogs agree in indicating a negative balance as a characteristic of the 

 first half of gestation, and that it is difficult to believe that the mere 

 transference of protein from the maternal to the embryonic organism by 

 whatever means before the complete establishment of the placenta should 

 occasion such marked and persistent losses of nitrogen in the urine, 

 particularly in view of the infinitesimal size of the embryo at this time, 

 and of the absence of other signs of a marked disturbance of the maternal 

 metabolism, particularly the basal metabolism. 



In direct opposition to the view that pregnancy is destructive of the 

 maternal organism, is the view developed from a number of somewhat 

 fragmentary observations on the nitrogen metaboHsm of human preg- 

 nancy, according to which the nitrogen stored in pregnancy is in great 

 excess of the needs of the fetus for its own growth and for the growth of 

 its protective and nourishing membranes. A nitrogen reserve is thus 

 built up to carry the mother over the puerperium and the initial period 

 of lactation, removing the necessity for a sacrifice of the maternal tissues. 

 This view has been discussed and subscribed to by Wilson (9°) and has 

 received support from his experimental observations. 



