126 



PROTEIN METABOLISM 



[PT. m 



trichloracetic acid filtrate was biuret-free. The dipeptide nitrogen, 

 estimated by Henriques & Sorensen's method, did not account for 

 more than a small proportion of it, nor by silver precipitation or other 

 means was it possible to arrive at a clear idea of its nature. 



Table 158. Lindsay s figures for percentage of the total nitrogen. 



One rather obvious consideration seems not to have been taken 

 into account by Lindsay, namely, that products of the foetal protein 

 metabolism are constantly passing through the placenta into the ma- 

 ternal circulation, even in the mammals with epitheliochorial placentas 

 studied by her. There is therefore no guarantee that, when we de- 

 termine the nitrogen partition in the early allantoic liquid, we are 

 determining the nitrogen partition of the total nitrogen excretion of 

 the embryo. Furthermore, it is obviously possible that if we knew 

 exactly how much nitrogen the embryo was excreting and in what 

 forms, through the placenta as well as through its own kidney, we 

 should find such substances as amino-acids and Lindsay's unidentifi- 

 able fraction to be much less important quantitatively than would 

 appear from her figures. And, on the other hand, there is the further 

 possibiHty, though it may be remote, that the walls of the allantois 



