SECT. 9] PROTEIN METABOLISM 1097 



allantoic fusion. These first appear about the time that the lymphatic 

 circulation is established in the allantois (7th day), and increase in 

 number and size till the end of development. Gentle heating coagu- 

 lates the liquid inside the blisters. They may either be concerned with 

 the absorption of protein, or they may be caused by the sharp edges 

 of the uric acid crystals, or, again, they may secrete mucin. As 

 regards the excretion of urates, Fiske & Boyden pointed out that 

 the anatomical "evidence" that the mesonephros is functionally 

 inert is no more than an expression of opinion, and that all we know 

 about the presence of nitrogenous waste products in the allantois 

 goes to show that it is functional. That water passes through the 

 mesonephros even against pressure as early as 2*5 days of incubation 

 was proved by the experiments of Boyden, who obstructed the 

 Wolffian duct, and afterwards observed hydronephrosis. Indeed, 

 without the mechanical distention produced by the excretion of the 

 mesonephros the allantois fails to grow normally in size, and con- 

 sequently the proper respiratory apparatus of the allantoic vessels 

 does not develop. And the mesonephros is alone in a position to 

 dispose of waste products up till the nth day. Aggazzotti, in his 

 work on the pH of the allantoic liquid (see Fig. 211), concluded 

 that the allantoic liquid was not a true excretion before the i ith day, 

 because it was not acid, although the yolk was, but, as Fiske & Boyden 

 point out, no one could have predicted a priori what the reaction of 

 the embryonic excreta would be. 



Fridericia was much interested in the problem of relating mor- 

 phological knowledge about such organs as the liver and the meso- 

 nephros to the results obtained on uric acid excretion. For the 

 former organ he could find no hint in the literature, but he made an 

 attack on the mesonephros by measuring its dimensions in a large 

 number of embryos. In the chick embryo the mesonephros is a large 

 yellowish-red organ on the 1 6th day, but as it hands over its functions 

 to the metanephros at a later stage it atrophies, and by the 20th 

 day is small, thin and pale. Fridericia's measurements of its size 

 are shown in Fig. 333, from which it appears that the mesonephros 

 reaches its greatest size upon the i6th day. Fridericia related this 

 to the peak which his figures for daily uric acid excretion show at 

 that point of development, but, as the only other set of data covering 

 it do not show such a falHng-off (Needham), it is doubtful if any 

 emphasis can be laid upon it. 



