SUMMARY 



Partial iransUion from aerobic to anaerobic metabo- 

 lism. 



1. In nature, partial transition from aerobic to anaero- 

 bic metabolism takes place in many invertebrates when 

 the oxygen tension in the surroundings drops below a 

 certain level. The critical oxygen tension at which such 

 a change occurs varies from species to species. 



2. The evidence for an actual transition to a partly 

 anaerobic metabolism under reduced oxygen tensions is 

 based on: (a) the excretion or accumulation of end prod- 

 ucts of anaerobic metabolism, (b) an increase in the rate 

 of carbohydrate consumption, (c) an increase in the res- 

 piratory quotient and (d) the accumulation of an oxy- 

 gen debt during the stay in the oxygen-deficient medium. 



3. The invertebrates are frequently divided into two 

 groups according to their reaction to lowered oxygen ten- 

 sions. In the first group the oxygen consumption re- 

 mains constant over a wide range of tensions, while in 

 the second it drops when the tension is still high. A 

 strict distinction between these two groups is, however, 

 not possible since the critical oxygen tension at which 

 the oxygen consumption of a given species begins to de- 

 cline is not fixed but depends largely on external and 

 internal factors. 



4. Experimentally, a partial transition to anaerobic me- 

 tabolism can be induced, not only by lowering the oxygen 

 tension of the surroundings, but also by eliminating a 

 large part of the aerobic processes by means of various 

 respiratory poisons and, occasionally, by changing the 

 salinity of the medium. 



Aerobic fermentations. 



1. ''Aerobic fermentations," that is, fermentations in 

 the presence of an abundant supply of oxygen, have often 



