252 GASTRULA-ARRESTED EMBRYOS 



is oxidized by a system of intermediate steps of the type usually 

 called a Kiebs cycle. 



Let us imagine, then, that events of the types diagrammed in 

 Fig. 13 occur both in R. pipiens and in hybrid embryos. Further- 

 more, for simplicity, let us make the possibly counterfactual as- 

 sumption that glycogen is the sole endogenous energy source of 

 pipiens and hybrid gastrulae, and on this basis let us try to local- 

 ize reactions whose subnormal rates may account for the bio- 

 chemical deficiencies just reviewed. From Fig. 13 it will be seen 

 that there are four major places, 1, 2, 3, and 4, at which partial 

 blocks may or may not occur. If S; is a blocked reaction at place i 

 and Si is a nonblocked reaction at place i, then we shall have 2^ 

 = 16 possible situations that may be the state of affairs in hybrid 

 gastrulae : 



Of these possibilities, only five are clearly disconfirmed by the 

 available data: (4) and (8), because if they were true, then the 

 anaerobic lactic acid production of hybrid gastrulae ought to be, 

 but is not, rate-normal; (12), because if it were true, hybrid an- 

 aerobic lactic acid production ought to be, but is not, rate-normal, 

 whereas aerobic lactic acid production ought to proceed at an ab- 

 normally high rate, but does not; (14), because if it were true, 

 then the oxygen consumption of hybrid gastrulae ought to be, but 

 is not, normal in rate; and (16), because differences between hy- 

 brid and normal embryos in respect to O2 uptake, lactate produc- 

 tion and glycogen utilization should not be, but are, observed. 



Several other possibilities— (1), (2), (3), (5), (7), (9), (10), 

 and (11) — are neither confirmed nor disconfirmed by existing 



