J. R. GREGG 251 



close together in a dish of aerated culture medium may excrete 

 considerable amounts of lactic acid, possibly because crowding 

 limits the rate at which environmental oxygen is supplied to 

 them. In nitrogen, both sorts of embryos accumulate large 

 amounts of lactic acid, hybrids to a lesser extent than R. pipiens 

 controls (Fig. 12). Thus, under anaerobic conditions, hybrid 

 embryos produce subnormal amounts of lactic acid. 



Localization of Metabolic Blocks in Hybrids 



From the foregoing sections, it is clear that hybrid gastrulae 

 exhibit subnormal oxygen uptakes, subnormal endogenous carbo- 

 hydrate utilization, and subnoniial anaerobic lactic acid produc- 

 tion. In discussing these results we shall make the following 

 assumptions about the intermediary carbohydrate metabolism of 

 hybrid gastrulae: 



1. Glycogen, in the presence of ATP (adenosine triphosphate) 

 and inorganic phosphate, is enzymatically oxidized to pyruvic 

 acid, with the production of hydrogen ions and ATP. 



2. Under anaerobic conditions, pyruvic acid is enzymatically 

 reduced by hydrogen ions and is converted to lactic acid, 



3, Under aerobic conditions, pyruvic acid is enzymatically oxi- 

 dized to carbon dioxide, with the production of ATP and hydro- 

 gen ions. 



4, Under aerobic conditions, hydrogen ions enzymatically re- 

 duce respiratory oxygen, with the formation of water. 



The exact mechanisms by which these events occur will not be 

 critical for the present discussion, but there is no reason to sup- 

 pose that they are unorthodox. Cohen (1954) has recently pre- 

 sented convincing evidence that glycolysis in R. pipiens embryos 

 is of the usual Embden-Meyerhof type, which, in the absence of 

 oxygen, terminates in lactic acid. Hydrogen transport and the 

 oxidative degradation of pyruvate have not been systematically 

 studied in R. pipiens or other amphibian embryos, but there is a 

 comparatively large scattered literature whose review is beyond 

 the scope of this contribution but which is consistent with the 

 hypothesis that hydiogen transport in amphibian embryos is car- 

 ried out in the classic Warburg-Keilin manner and that pyruvate 



