ANAEROBIC METABOLISM 209 



gate's experiments were carried out in vitro, but it is 

 certain that the same type of metabolism prevails also 

 when the parasites live within the intestine of their hosts, 

 as the following observation of Cook (1932) indicates. 

 He noticed that the termites harboring their normal in- 

 testinal fauna produced a gas that was not absorbed by 

 strong alkali (and which must therefore be identified 

 with the hydrogen found by Hungate), while defaunated 

 ones did not, a clear indication that, in vivo, the hydro- 

 gen formation is also due to the intestinal protozoa. Ad- 

 ditional data on this hydrogen formation will be found 

 in the paper by Cook and Smith (1942). 



Another case of mixed fermentations has been de- 

 scribed for the ciliate Tetraliymena gelei. Thomas (1942) 

 identified lactic, acetic and succinic asids as end products 

 of the anaerobic carbohydrate metabolism of this or- 

 ganism. He considers the formation of the lactic acid 

 as a process unrelated with those leading to the other 

 acids, and, as already mentioned above, he is convinced 

 that the lactic acid results from intermediate steps iden- 

 tical with those found in vertebrate muf^cle. His assump- 

 tion of the independence of the two processes finds some 

 support in his observation that the proportions in which 

 the various substances are formed are variable, depend- 

 ing upon the medium in which the experiments are con- 

 ducted. In phosphate buffer lactic acid predominates, 

 while in bicarbonate media succinic acid is more abun- 

 dantly formed. 



The formulas (c/. Table 24) by which Thomas repre- 

 sents the formation of acetic and succinic acids are of 

 considerable interest. Especially significant is the way 

 in which the succinic acid is formed, since it is a process 

 requiring an assimilation of carbon dioxide from the me- 

 dium. That this actually happens has been proven beyond 

 doubt. Thomas (cf. also van Niel, Thomas, Euben and 

 Kamen, 1942) used radio-active carbon as a tracer and 



