SECT. 7] OF EMBRYONIC DEVELOPMENT 987 



example, Liebermann had shown that the fat of the egg contained 

 71-67 per cent, of carbon, and the amount used per egg Tangl & 

 von Mituch found to be on an average 2-11 gm. According to Hassel- 

 balch, the total amount of carbon dioxide produced during incuba- 

 tion amounted to 5-939 gm. From this they calculated the amount 

 of fat which ought to have been used, and the result was 2-26 gm., 

 which agreed moderately well with Liebermann's figure, and with 

 one which they themselves found, 2-76 gm. Another line of investi- 

 gation which gave support to this view was the work of Bohr & 

 Hasselbalch on the respiratory quotient of the hen's egg, which gave 

 from the 7th day onwards a figure of 0-73. The following curious 

 argument occurs in Liebig's Animal Chemistry of 1842, and shows once 

 more the same point of view. "The egg of a fowl can be shewn to 

 contain no other nitrogenised compound except albumen. The albumen 

 of the yolk is identical with that of the white, the yolk contains besides 

 only a yellow fat in which cholesterine and iron may be detected. 

 Yet we see in the process of incubation, during which no food and no 

 foreign matter except the oxygen of the air is introduced or can take 

 part in the development of the animal, that out of the albumen feathers, 

 claws, globules of the blood, fibrine, membrane and cellular tissue, 

 arteries and veins are produced. The fat of the yolk may have 

 contributed to a certain extent to the formation of the nerves and the 

 brain, but the carbon of this fat cannot have been employed to produce 

 the organised tissues in which vitality resides because the albumen of 

 the white and yolk already contains for the quantity of N present, 

 exactly the proportion of carbon required for the formation of these 

 tissues." 



It might always have been doubted, however, that fat was the 

 only important source of energy: there were hints to the contrary in 

 the literature. William Harvey had said, "and therefore the yolke 

 seems to be a remoter and more deferred entertainment than the 

 white, for all the white is quite and clean spent before any notable 

 invasion is made upon the yolke". Another important observation 

 was that of Prevost & le Royer in 1825, '^^o obtained from the 

 allantoic fluid of a 1 7-day old chick a nitrogenous substance which 

 gave an insoluble nitrate and resembled urea in all particulars. There 

 had clearly been some catabolism of proteins. And since at about 

 the same time Jacobson; Sacc; and Stas found uric acid in consider- 

 able amounts in the allantoic fluid of developing chicks during the 



