SECTION 8 

 CARBOHYDRATE METABOLISM 



8-1. General Observations on the Avian Egg 



Our knowledge of the carbohydrate substances in the egg of the 

 hen begins, where so much embryological history begins, with 

 Wilham Harvey, who in the De Generatione Animalium says, "Egges 

 after two or three daies incubation are even then sweeter rehshed 

 than stale ones are. And after full fourteen daies (when the Chicken 

 now beginneth to be downey and extendeth his Dominion over halfe 

 the eggc and the yolke is almost still entire) I have boyled an egge 

 till it was hard that so I might discerne the position of the chick 

 more distinctly — and yet the yolke was as sweet and pleasant as that 

 of a new laid Egge when it is likewise boyled to an induration". 

 Compare with this, Fig. 275. 



The subject of the carbohydrate metabolism of embryonic life is 

 not a difficult one to deal with if a few initial propositions are kept 

 in mind. Thus, as will appear later, glycogen cannot be considered 

 as a representative carbohydrate, although some workers have 

 assumed that it is. Then the earlier work has all to be judged in the 

 light of the fact that copper reducing methods, so universally used 

 for determining glucose quantitatively, give results which are far too 

 high in the presence of protein breakdown products. The earlier work 

 can accordingly be used only when there is reason to believe that 

 peptides and amino-acids were excluded, as in the determination of 

 free glucose after dialysis or precipitation of proteins, and not when 

 the estimations were carried out on protein hydrolysates. Holden 

 found that the Hagedorn-Jensen method, which involves the reduction 

 of ferricyanide not of copper, was the most reliable, and the sub- 

 sequent work of Pucher & Finch; Duggan & Scott; Fazekas; and 

 Jonsell, Jorpes & Sikstrom has demonstrated the same thing. Passing 

 now from questions of technique, it will be best to use our knowledge 

 of the carbohydrate metabolism of the hen's egg as the skeleton on 

 which to build up this chapter, for it is by far the best known case, 

 and possesses all the important features, of embryonic carbohydrate 

 metabolism. 



