SECT. 9] 



PROTEIN METABOLISM 



1127 



themselves contribute nitrogenous substances to the liquid within 

 them. Lindsay's assumption, in fact, that the composition of the 

 allantoic liquid in the early stages gives an undistorted picture of the 

 foetal protein metaboHsm must be admitted only with reservations. 



In this connection it is worth while anticipating the chapter on 

 placental permeability to allude to the work which has been done 

 on the concentration of nitrogenous end products in the maternal 

 and foetal blood. By studying them, many investigators have hoped 

 to discover the existence of a concentration gradient between the 

 foetal and maternal organisms. The relevant figures are as follows (all 

 on human blood) : 



Table 159. 



Urea nitrogen (mgm. %) Uric acid nitrogen (mgra. %) 



Slemons & Morriss observed that rises and falls in the maternal 

 blood urea were always accompanied by like rises and falls in the 

 foetal blood urea, and they concluded that urea passed into the 

 maternal circulation by diffusion. As the above table shows, the 

 concentrations of nitrogenous end products in the maternal and 

 foetal blood are almost identical, with a sHghtly higher level in some 

 cases in the foetal blood. If a gradient exists, then, it is in the 

 direction foetus ->mother. Kreidl & Mandl in 1904 summarised the 

 reasons for believing that by far the greater part of the nitrogenous 

 excretion of the foetus passes through the foetal kidneys. The in- 

 teresting anatomical evidence for this, with its bearing on the function 

 of the mesonephros, will be considered in the Section on placental 

 permeability. 



If, then, we cannot suppose that the nitrogenous end products 

 which collect in the allantoic sac of the herbivorous mammal are 

 the sum of all the nitrogenous end products of the embryo, there is 

 not much use in relating them to its total weight, i.e. in attempting 



N E II 72 



