CHANGES DURING PREGNANCY 521 



both the experimental data which formed the basis of the pathological 

 conception of pregnancy and the arguments on which it was built up 

 were open to criticism. 



In a normal mother carrying a healthy foetus and living on an 

 adequate diet, pregnancy does not entail a sacrifice on the part of the 

 mother. In the words of Bar : " L'etude de la balance azoto>e n'autorise 

 pas a dire qu'en principe le gestation constitue une periode de 

 sacrifice; elle montre au contraire qu'elle pent etre et qu'elle est 

 souvent une periode de profit. La gestation apparait olonc comme un 

 exemple de symbiose harmonique hornogene." 



The various aspects of the metabolism of pregnancy \vill now be 

 discussed in greater detail. 



A. The Source of the J/te/-m/.s- traiisf erred to //' Xi-iv Organism 



The question is discussed in another chapter (see p. 509) whether 

 the materials that go to the formation of the new organism are 

 elaborated entirely in the new ovum itself, or are wholly prepared by 

 the mother. As stated there, the histological and biological evidence 

 leads us to. believe that the materials, whatever their source and 

 ((institution, are in the early stages broken down and partly re- 

 synthesisetl by the trophoblast, while later in pregnancy they are 

 metabolised by the fojetal cells themselves. Granting this, we must 

 further suppose that the maternal duties do not extend to the formation 

 of fetal tissue-components, but are limitetl to the provision of food 

 and oxygen for the fertilised ovum, the removal of- its waste products 

 and the preparation of an organ of nutrition for the new-born young. 

 Each of these duties leads to changes in metabolism, which may, in 

 addition, be exciteol by special stimuli produced during pregnancy 

 (see p. 515). 



In the provision of nutriment for the embryo, does the mother 

 deplete her own tissues, or is she content to transfer the unorganised 

 substances which are absorbed from the t'ooil and not yet fixeol as 

 vital constituents of the protoplasm .' Probably both. In insuHicient 

 nutrition the mother certainly gives up organised tissue-products, 

 and even with a plentiful diet a period lias been observed in the dog 

 in the earlier stages of pregnancy (see p. ~>27) during which the 

 maternal organism loses nitrogen and therefore draws on its own 

 tissues. On the other hand, in the later stages nt' pregnancy when 

 the requirements of the fo>tus are greatest there is on an adequate 

 diet a considerable retention of nitrogen. This is brought about, as 

 will lie seen, by a more perfect assimilation of the absorbed food. At 

 this stage, therefore, the unorganised substances of the absorbed food 

 are apparently being utilised directly by the trophoblast. This 

 17 A 



