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 fcEtus 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 azot^e n'autorise 

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

 sacrifice; elle montre an contraire qu'elle pent §tre et qu'elle est 

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

 exemple de symbiose harmonique homogene." 



The various aspects of the metabolism of pregnancy will now be 

 discussed in greater detail. 



A. The Source of the Materials transferred to the New Orga^iism 



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 

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

 synthesised by the trophoblast, while later in pregnancy they are 

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

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

 of foetal tissue-components, but ' are limited 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 excited 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 food and not yet fixed as 

 vital constituents of the protoplasm ? Probably both. In insufficient 

 nutrition the mother certainly gives up organised tissue-products, 

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

 in the earlier stages of pregnancy (see p. 527) during which the 

 maternal organism loses nitrogen and therefore draws on its own 

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

 the requirements of the foetus are greatest there is on an adequate 

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

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

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

 are apparently being utilised directly '^ly the tropholilast. This 



17 A * 



