Glycogen Turnover 305 



tion of epinephrine, the animal is not demonstrably sick and indeed 

 under most circumstances its chance of survival has been but little 

 diminished when compared with that of a litter mate whose liver is 

 rich in glycogen. Lastly, the major process of glycogenesis, the phos- 

 phorylase reaction, is the precise reversal of the major process of 

 glycogenolysis. In other words the route in is the reversal of the route 

 out ; hence glycogen is a sort of biochemical blind alley. In summary, 

 the reserve of liver glycogen is small, the animal does quite well with- 

 out it, and it is a cul-de-sac. It will be recalled that on these very 

 grounds the vermiform appendix in man has been designated as a 

 vestigial organ. 



Is liver glycogen then to be considered a biochemical vestige? The 

 probable answer is in the negative, in view of the fact that under 

 certain stresses of short duration, liver glycogen does serve as a useful 

 source of extra blood glucose. The fact remains, however, that, 

 whereas the well-nourished potato accumulates polysaccharide, the 

 well-nourished rat accumulates characteristically not glycogen but 

 lipid. The major elastic compartment for energy storage in adult 

 mammals is the depot fat, not the carbohydrate compartment. This 

 fact was drawn to our attention some years ago when we had occasion 

 to compare rates of glycogenesis and lipogenesis in adult and fetal 

 rats. 15 Although the fetus is, until shortly before term, very poor in 

 depot fat, its glycogenic capacity, per gram of tissue, is far greater 

 than that of the adult. The possibility that this difference in fetal and 

 adult habits might represent a sort of recapitulation of phylogeny 

 suggests itself. 



In quest of a more satisfactory explanation for the lack of fat storage 

 in fetal tisues, one is forced to consider the generally stated functions 

 of subcutaneous depot fat. This adipose tissue serves as an energy 

 reservoir, it functions as mechanical upholstery against physical 

 trauma, and it acts as thermal insulation in homothermic species. For 

 none of these functions does the mammalian fetus have any need. 

 It is nourished not discontinuously but continuously by the maternal 

 circulation. It is well protected against physical trauma from without 

 by maternal tissues and amniotic fluid. It resides in a precisely 

 thermoregulated environment. It is therefore not surprising that the 

 fetus receives its investiture of subcutaneous adipose tissue only shortly 

 before term, as if in anticipation of its ejection from its highly pro- 

 tected environment. 



A further inspection of the difference in energy-storage habits of 



