SECT. 8] CARBOHYDRATE METABOLISM 1035 



insulin from the foetal pancreas. Such experiments are open to 

 various criticisms on grounds of technique, and in Section 21 '8 we 

 shall examine them in more detail. They found that up to the 7th 

 week there was no protection but that it was demonstrable afterwards. 

 This agreed chronologically with the time at which the transformation 

 of Laguesse islets into Langerhans islets was going on, and suppHed 

 more evidence that the former were incapable of secreting insulin. 

 Then Potvin & Aron investigated the islet tissue in the pancreas of 

 the chick. They found that the first islets appear on the 8th day of 

 development, and for a day or two those of Laguesse predominate, 

 but these soon give place to Langerhans islets, so that by the 15th day 

 there are none of the former left. It is impossible not to be struck 

 with the correlation between these histological and physiological data, 

 and the biochemical evidence of Figs. 284 and 285. 



8-7. General Scheme of Carbohydrate Metabolism in the 

 Avian Egg 



A general scheme of carbohydrate metabolism in the developing 

 chick may therefore be provisionally summarised as follows: 



{a) Zero hour of development till the yth day. The carbohydrate in the 

 embryo at the earliest stages, i.e. about the time when the yolk is 

 first completely enclosed by the blastoderm, is to a very large 

 extent composed of glucose in combination with protein. Glycogen 

 increases outside the embryo, being stored in the yolk-sac and parts 

 of the blastoderm. Free sugar increases steadily within the embryo 

 both absolutely and relatively, being uncontrolled by the pancreatic 

 hormone. The liver and pancreas arise as epithelial buds from 

 the wall of the duodenum on the 3rd day. Glucose combustion 

 predominates. 



{h) Eighth to nth day. The importance of the mucoprotein fraction 

 of the glucose in the embryo diminishes, but the free glucose con- 

 tinues to rise proportionately, as does the glycogen in the yolk-sac 

 and blastoderm. At this time 90 per cent, of the egg's glycogen is 

 outside the embryo. The liver and pancreas change their growth, the 

 former increasing in bulk and the latter developing the islets of 

 Laguesse. By the loth day the islets of Langerhans are in evidence, 

 and insulin is being secreted in increasing amounts. At this time 

 also a certain extra quantity of fat enters the embryo from the yolk, 

 and is there transformed into carbohydrate. 



