I032 CARBOHYDRATE METABOLISM [pt. iii 



for it to do so, for a preponderance of glycogen over free glucose may 

 roughly be taken as characteristic of the mature animal. Another 

 instance of such a comparatively rapid passage from the embryonic 

 to the adult condition will be seen in the nitrogen partition in the 

 urine of the chick (Section 9-5). 



It is interesting to note that the effect of insuHn on the free glucose 

 curve seen in Fig. 284 is a phenomenon revealed in the intact 

 organism following its usual courses and not subject to the abnor- 

 maUties necessarily consequent upon depancreatisation or other 

 experimental treatment. Another point is that the appearance of 

 insulin occurs in that portion of embryonic life in which development 

 is irrevocably determined and self-differentiation is going on. During 

 the early stages, when a great degree of regulation is possible, the 

 embryo has no insulin, and the hormone only arises after chemo- 

 differentiation has taken place and the fate of every cell is finally 

 determined. As will appear in Section 15, good evidence also exists 

 that this applies to other hormones besides insulin, and probably 

 to all of them. 



Hanan demonstrated that insulin hypoglycaemia can be produced 

 during the last week of development. Taking 14-16-day White Leg- 

 horn embryos, he injected insulin into the air-sac, and then, with- 

 drawing blood from the allantoic vein at its bifurcation just below 

 the air-sac, estimated the blood sugar by the Hagedorn-Jensen method. 

 From the normal value of 209-296 mgm. per cent, a marked lowering 

 could be observed. If glucose was injected into the air-sac instead of 

 insuHn, the blood sugar rose, and it could also be made to rise by 

 bleeding the egg. Riddle's finding that adult birds will survive a dose 

 of insulin thirty times as strong as that which would kill a rabbit 

 was extended by Hanan to this later period of incubation. 



In 1922 Aron, as the result of comparative studies on the embryos 

 of the sheep, guinea-pig, pig, and man, made the generalisation that 

 the glycogenic function of the liver (as judged by the accumulation 

 of glycogen within it) always occurred at the moment when the 

 interstitial portion of the pancreas was taking on its adult histological 

 appearance. His experiments did not include the determination of 

 the glycogen content of the placenta, but he analysed the foetal liver 

 at different stages of development, and obtained the curves shown in 

 Fig. 286. "The glycogenic function of the liver", said Aron, "mani- 

 fests itself at a fixed time in ontogeny. Its installation varies in different 



