SECT, ii] FAT METABOLISM 1171 



acids of the chick embryo and the rest of the egg throughout develop- 

 ment. Her resuks are plotted in Fig. 365, from which it appears that 

 there is little change during the first 10 days of development, but 

 that after that time the iodine value of the fatty acids in the chick 

 rises, while that of the fatty acids in the remainder of the egg falls. 

 This decrease in the iodine value of the yolk-fat during development 

 is highly suggestive of a primary absorption of the less saturated fats 

 with a subsequent equal absorption of saturated and unsaturated fat. 

 The increase in the iodine value of 

 the embryonic fatty acids points to 

 an absorption of the more desatu- 

 rated fatty acids, and suggests that 

 in the early stages the chick embryo 

 does not possess the power of per- 

 forming the desaturation. If the 

 embryonic liver was incapable of 

 desaturating fatty acids until, say, 



(O Chick 

 Eaves <• Remainder 

 (3 Whole egg 



Days-*-5 To is 20" 



the loth day of development, it P^ g 



might be supposed that the unsatu- 

 rated fatty acids would be preferentially absorbed, as the saturated 

 ones would be of no use to the embryo. However, no a priori argu- 

 ments are here of importance, and I accordingly arranged a series of 

 experiments to test the hypothesis. An aqueous emulsion of embryonic 

 tissues was mixed with the corresponding yolk and vigorously shaken, 

 after which it was allowed to stand anaerobically under toluene for 

 5 days at 37°. The results were as follows (fatty acids being separated 

 by the Mottram-Lemeland technique) : 



Thus where the 12th and 19th day embryonic cells were present 

 there was a very marked desaturation occurring, but not in the case 

 of the 6th day embryo, nor the 6th day yolk alone. If this set of 

 experiments should be confirmed, it will show that in the earlier stages 

 the embryo does not possess the power of desaturating the fatty acids 

 it absorbs from the yolk. It is therefore forced to use the unsaturated 



