io84 PROTEIN METABOLISM [pt. m 



to McDowell's log. weight/age graph described in Section 2-2 and 

 illustrate well the differential growth-rates of the three functions. 

 Ammonia nitrogen appears first but grows slowly and is soon over- 

 taken by urea nitrogen, which in its turn is overhauled and passed 

 by the uric acid nitrogen "growing" very rapidly, and finally 

 reaching much larger proportions than any of the others. The graph 

 also shows that finally the embryo has produced about ten times as 

 much urea nitrogen as ammonia nitrogen and about ten times as 

 much uric acid nitrogen as urea nitrogen. 



There seems no reason to hesitate in classing this sequence among 

 the most remarkable instances we possess of the occurrence of a 

 recapitulation phenomenon in chemical embryology. To designate it 

 thus does not, however, explain it and I shall return to the theory of 

 recapitulation in the Epilegomena. The recent researches of Przylecki 

 & Rogalski have thrown some light on the sequence of nitrogenous 

 excretory compounds in the chick from another angle. In connection 

 with Przylecki's wide investigations concerning the manner of excret- 

 ing nitrogen which characterises different phyla and species, he 

 directed his attention to the ontogenetic side, and enquired whether 

 the development of the embryo involved the appearance or disappear- 

 ance of such enzymes as uricase. For this purpose Przylecki & Rogalski 

 used chick embryos to find out when the embryonic tissues can form 

 uric acid, and when they can break it down. For the first series 

 embryos were minced to an emulsion in water, glycerine and chloro- 

 form, and to some flasks xanthine was added, to others not. The results 

 are shown in Table 139. Obviously if xanthine oxidase were present 

 and active, the flasks to which the xanthine was added should show in 

 all cases a higher uric acid content at the end of the experiment than 

 those which had received no such addition. Here there is a turning- 

 point between the 4th and 7th days of development, for before that 

 time the xanthine flasks have no advantage over the others, but after 

 that time an advantage is found without exception. This change- 

 over point occurs at exactly the same time as the sudden rise 

 of the control, which up to the 7th day reveals no preformed uric 

 acid in the experimental material. The conclusion to be drawn then 

 is that at some time between the 4th and the 7th day the embryonic 

 xanthine oxidase awakens into activity, and the embryo acquires the 

 power of forming uric acid from xanthine. If these time relations are 

 compared with the appearance of uric acid shown in Fig. 317, the 



