iio6 PROTEIN METABOLISM [pt. iii 



0-076 mgm., or 5-6 per cent, of the initial dry weight. They next 

 estimated the nitrogen in the egg at the beginning and in the hatched 

 tadpole at the end, using the micromethods of Pilch and Bang, and 

 obtaining the following averages : 



Milligrams nitrogen per 100 embryos 



One hundred embryos during their pre-natal life, then, lost i • 1 9 mgm. 

 of nitrogen, or g-i per cent, of the initial quantity provided. After 

 hatching this decomposition of protein still goes on, as Fig. 336, taken 

 from Bialascewicz & Mincovna's figures, shows. Turning now to the 

 determination of the end products, the total nitrogen excreted by 

 the embryos into the surrounding water was estimated, and related 

 to a standard number of embryos in 24-hour periods. This led to 

 the striking curve in Fig. 337, which shows the relative intensity of 

 nitrogen excretion from frog embryos, and as it is an intensity graph 

 it can be compared directly with that for the chick as given in Fig. 325. 

 To express the amphibian production of nitrogen in milligrams per 

 cent, dry weight of embryo is not possible, as the embryo cannot be 

 dissected away from the yolk. We can only tell what 100 mgm. 

 dry weight of frog embryo-plus-yolk excretes per day, but, as the 

 total change of dry weight throughout development is not more 

 than 6 per cent., there is no necessity to turn the excretion curve 

 into terms of dry weight, and it is sufficient to have it in terms of a 

 constant number of embryos. It is obvious that, if we could express 

 it in terms of "live" dry weight, the peak would be much more in 

 evidence than it is, for the "live" dry weight is continually increasing, 

 and the "dead" or yolk dry weight is continually decreasing — there- 

 fore the descent after the 125th hour would be emphasised consider- 

 ably. It may be said, then, that in the frog, as well as in the chick, 

 there exists a period towards the middle of embryonic development 

 at which more protein is combusted than at any other time. The 

 observation of this peak in a bird and an amphibian would suggest 

 that the phenomenon is a general one. 



