502 THE PHYSIOLOGY OF REPRODUCTION 



still unproven. Such is undoubtedly the case on a minimal 

 protein diet ; but, with a greater allowance of nitrogen, the con- 

 clusions of Hagemann and the others are not borne out by the 

 more recent researches of Bar and Daunay. They fed three 

 pregnant bitches on a constant diet of bread, water, fat, beef, 

 and salt, and estimated the nitrogen of the urine and faeces at 

 regular intervals. They took precautions to secure the young 

 and the after-births, and were able to determine accurately 

 their nitrogenous content. In the three animals, as in two 

 observed by Jagerroos, the period of gestation was triphasic. 

 There were first a period of retention of nitrogen, then a balance 

 or very slight loss, and finally a retention increasing with the 

 progress of gestation. Further, there was over all a gain of 

 nitrogen in two dogs at the conclusion of labour. Hence they 

 conclude that pregnancy in a healthy animal, with a rational 

 and sufficient diet, does not necessitate a drain on her stock of 

 nitrogen to satisfy the needs of the foetus. 



Jagerroos showed that, on a minimal protein diet, the 

 nitrogenous equihbrium was disturbed by the onset of pregnancy. 

 The loss of nitrogen began during the first week after conception, 

 when the fertihsed ova were still in the oviduct or had just 

 reached the uterine cavity. At this stage the daily fixation of 

 nitrogenous substances in the yoimg blastocyst is too small 

 in amount to affect per se the nitrogenous equihbrium. At the 

 same time there is an appreciable daily loss of nitrogen which 

 must be derived from the maternal tissues. It is permissible to 

 assume a relationship between the two facts, and argue that 

 the presence of the young blastocyst leads in some way to an 

 increased katabohsm of protein.^ The daily loss of tissue is 

 maintained for a longer or shorter period during pregnancy, 

 according as the nitrogenous content of the diet is near the 

 minimum or more abundant. But, even on a comparatively 

 rich protein diet, it is generally accepted by the investigators 

 that a negative balance occurs for a considerable period, that 

 it then disappears, and is replaced by a positive balance lasting 



^ It must, however, be kept in mind that the corpus luteum is undergoing 

 active changes at this period. As already stated, it is not known whether 

 the ovum or the corpus luteum provides the stimulus for the changes 

 referred to, or whether both of these are concerned (see p. 504). 



