SECT. 6] GENERAL METABOLISM 929 



may be seen that the absorption intensity is then at its highest. In 

 this way carbohydrate differs from protein and fat. 



During the first 6 days the absorption of carbohydrate and protein 

 (both "water-soluble substances") is very high, while that of fat is 

 very low. From the 6th to the 13th days the intensity of absorption 

 of fat is high, and protein and carbohydrate are low. From the 

 13th to the 17th days the absorption of fat again drops, and that of 

 protein rises, but carbohydrate does not accompany it; on the con- 

 trary, it remains very low, though moving slowly in an upward 

 direction. At the i8th day there is another sharp cross-over between 

 protein and fat which is not shared by carbohydrate. 



The behaviour of carbohydrate gives, therefore, some support to 

 the theory that changing types of permeability in the yolk-sac 

 cells are responsible for these effects. In the earlier periods when the 

 absolute amounts of carbohydrate being absorbed are comparable 

 with the absolute amounts of fat and protein being absorbed, the 

 protein-carbohydrate curve does tend to be the reciprocal of the fat 

 curve, but later on the carbohydrate drops out of the relationship, 

 and pursues an uneventful course of its own. Thus on the 6th day 

 the embryo absorbs 5 mgm. of carbohydrate, 6 of protein and 2 of 

 fat, while on the 1 6th day it absorbs 1 1 of carbohydrate, 449 of 

 protein and 333 of fat. During the period, then, when the absorption 

 of carbohydrate is at all equivalent to that of the other food-stuffs 

 the relations predicted by the hypothesis hold in practice. 



It will be worth while to compare carefully the curves for absorp- 

 tion intensity in the chick embryo, with those for its constitution, 

 for the correlation between constitution and absorption is quite 

 close. When the intensity of absorption of carbohydrate is at 

 its highest, then there is more carbohydrate in the embryo than at 

 any other time. When the intensity of absorption of protein is passing 

 its second peak the constitution curve has also its peak. When the 

 intensity of absorption of fat has risen to its highest level the amount 

 of fat in the embryo has done so too. 



On the other hand, there are two points where the correlation is 

 not obvious. The first peak on the fat absorption curve has no corre- 

 sponding inflection on its constitution curve, and the initially high 

 absorption intensity of protein has nothing to correspond with it. 

 There is, of course, no a priori reason to expect close agreement 

 between the peaks on the absorption and constitution curves. 



