J. R. GREGG 253 



data. To evaluate these cases we shall need additional informa- 

 tion. In particular we shall have to know which of the blocked 

 reactions, if any, become rate-limiting for the entire system, and 

 the degrees to which oxygen and pyruvic acid can compete suc- 

 cessfully for hydrogen ions under each of the postulated condi- 

 tions. Consider case (1), for example, where there is imagined 

 a partial block at all four sites. Under these conditions we should 



GLYCOGEN 



ATP + 

 PYRUVIC <r 

 ACID — 



1 



-- 2 



LACTIC L. 

 ACID 



f-KREBS CYCLE<- 



INORGA^fIC 

 PHOSPHATE 



OXYGEN 



/N --3 



'— ^ WATER 



-> CO, + ATP 



Fig. 13. Carbohydrate metabolism schema. 



expect to find, and do find, that hybrids respire and utilize glyco- 

 gen at a subnormal rate and produce lactic acid anaewbically at 

 a subnormal rate. But we cannot predict how hybrid and R. pipiens 

 gastrulae will compare in respect to aerobic lactic acid produc- 

 tion, for this would depend, among other things, upon knowing 

 (as we do not) which of Si, S2, S3, and S4 are rate-limiting so that 

 we would know which intermediates accumulate, and how well 

 under these restrictions pyruvate and oxygen compete for hydro- 

 gen ions. Therefore, the fact that we actually do find the hybrids 

 aerobically producing lactic acid at a normal, negligible rate is 



