1 1 72 



FAT METABOLISM 



[PT. Ill 



ones, and possibly, as these are not present in great quantity, it is 

 thus prevented from devoting much fat to combustion. 



In a previous discussion of protein metaboHsm, the work of Riddle 

 on the yolk-contents in the latter half of incubation was cited, and 

 it is equally relevant here, for he determined the percentage (dry 

 weight) of neutral fat in it from the 12th day onwards, using the 

 method of Koch. His data, which are plotted in Fig. 366, demonstrate 

 that the percentage of fat in the yolk-sac falls towards the end of 

 development, just as the percentage of protein rises, indicating a pre- 



Vladimirov S^Schmidb 



5 Days 



5 Days 



Fig. 366. 



Fig. 367. 



ferential absorption of the former over the latter, and bearing out 

 the absorption intensity curves shown in Fig. 251. The rest of the 

 information is more difficult to understand, and concerns special 

 fractions of yolk; thus the intracellular yolk (yolk-masses in the wall 

 of the yolk-sac) seems to have a high fat concentration, and the 

 solid masses in the body of the yolk itself a variable one. As for 

 the yolk-sac, its fat-content rises greatly in the last few days of 

 development. We shall see later that this preferential absorption of 

 fatty acids is accompanied by a preferential absorption of phosphatides. 

 This picture of the awakening of fat metabolism in the last days of 

 incubation is to some extent mirrored in the results of Vladimirov 

 & Schmidt on the blood fat of the embryo. As Fig. 367 shows, it 



