SECT. 9] 



PROTEIN METABOLISM 



;o79 



ment the uric acid is exceedingly small in amount, but from the 7th to 

 the I ith day it rises rapidly, until on the 1 2 th day it attains a constant 

 level which it does not leave. There is thus a specially intensive pro- 

 duction of uric acid between the 7th and the nth days of incubation. 



Fig. 316 gives the uric acid in milligrams per cent, dry weight of 

 embryo. The curve reaches a peak on the nth day, after which it 

 descends, and seems to be reaching a steady level by the time of 

 hatching, at about 460 mgm. per cent. Its shape is the same as that 

 found previously for urea. In Fig. 3 1 7 are shown the urea and the 

 uric acid in milligrams per cent, wet weight of embryo plotted 

 on the same scale. It shows the interesting fact that, on the 3rd, 

 4th, 5th, 6th and 7th days of incubation, the uric acid is rising 

 distinctly more slowly than the 

 urea, and, indeed, in absolute '2001- 

 quantities per egg there is less 

 uric acid than urea until the 

 8th day is reached. Between 

 the 7th and the 8th day, the 

 uric acid rises tremendously in 

 amount, and, overtaking the 

 urea, almost attains its final 

 constant value. These relation- 

 ships are better seen in Fig. 

 3 1 8, which gives the miUigrams 

 per cent, wet weight for both 

 uric acid and urea, the abscissa being arranged so as to get them both 

 on to the same graph. When this is done, it is obvious that, though both 

 urea and uric acid rise in course of time to a constant level, the urea 

 starts rising much earlier than the uric acid, and reaches its maximum 

 level a day or so before. This is reflected on the milligrams per cent, 

 dry weight curve, shown in Fig. 319, and it is seen that the urea is 

 in advance of the uric acid by two days. 



In the hen, as we have seen, the excreted nitrogen is mostly in the 

 form of uric acid, and the urea takes only a very small share of it. 

 At what stage in embryonic development is the adult relationship 

 reached? In Table 137 and Fig. 320 the ratio uric acid/urea is seen. 

 From the 14th day onwards it is constant at about 16, that being the 

 adult level, but before the 7th day its value is less than unity, because 

 more urea is present and more urea is daily excreted than uric 



N E II 69 



Days 



Fig. 316. 



