630 VETERINARY PHYSIOLOGY 



amino-acids are the results of the protein metabolism of the 

 fcetus. 



Chemical examination of the allantoicfluid of ungulates, which 

 is foetal urine (p. 626), shows, in the early stage of development, a 

 high proportion of nitrogen in amino-acids, peptides and allantoin. 

 This seems to indicate a less complete catabolism of protein 

 and a more active nuclear metabolism than in extra-uterine life. 



The placenta manifests little power of regulating the 

 materials which are passed to the foetus, and most drugs and 

 toxic substances reach it. Even tlie micro-organisms which 

 are the cause of various diseases may pass through the 

 placenta. In this respect it seems to differ from the cells of 

 the choroid plexus, which manifest a selective action on the 

 materials which it passes to the cerebro-spinal fluid (p. 511). 



IV. Growth of the Fcetus. 



The growth of the foetus is steady and slow, aud the daily 

 demands on the mother are comparatively small. At birth the 

 human foetus is less than 8 per cent, of the weight of the mother, 

 while in some animals, e.g. the dog, the litter may weigh 20 or 

 25 per cent, of the maternal weight. 



Considering the fact that the power of fixing material in the 

 body is increased during pregnancy, the amount of the food 

 consumed which has to be transmitted to the foetus is com- 

 paratively trivial. 



Only in the later period of intra-uterine life is the 

 demand for proteins, and, more especially, for fats in any 

 way considerable. 



It may thus be said that under all conditions of normal 

 nutrition it is the surplus of nourishment which is passed from 

 the mother to the foetus, and, if in the later months of 

 pregnancy her nourishment is limited, the size of the young may 

 be reduced. 



The maternal tissues part more readily with some sub- 

 stances than with others. Thus the demands of the foetus for 

 calcium, when the supply to the mother is inadequate, may be 

 met by removal of calcium from her bones, which may thus be 

 softened. On the other hand, the maternal tissues do not 

 become depleted of iron in the same way to meet the require- 

 ments of the young. 



