METHODS OF INVESTIGATION 21 



amounts of oxygen even at low tensions, the strictest 

 precautions are necessary. To put, say, a worm of 5 g. 

 weight into 50 cc. of a solution containing 0.1 cc. of 

 oxygen per liter is one thing, while to put small rotatoria 

 or protozoa (even in large numbers) in the same solu- 

 tion would be quite a different proposition. If an in- 

 vestigation is concerned with the question whether a 

 certain process, for example, the cortical layer response 

 of the echinoderm egg, can go on without oxygen, it will 

 evidently be necessary to eliminate even the smallest 

 traces of oxygen as far as this is technically possible. 

 The same holds true if one investigates the possible in- 

 fluence of very small traces of oxygen on the survival of 

 animals. 



To remove even the last molecule of free oxygen that 

 is dissolved in the medium or in the tissues of the experi- 

 mental animal is an ideal obviously never realized. By 

 selecting the proper methods and applying them correct- 

 ly, however, one can at least approach this ideal condi- 

 tion. 



3. ANAEROBIC MEfTABOLISM. 



A complete study of anaerobic metabolism requires the 

 gathering of quantitative data about the changes that 

 occur in the chemical composition of the organism sub- 

 jected to anoxic conditions and in the medium in which 

 it is kept. The gaseous, liquid and solid substances ab- 

 sorbed or excreted must be determined quantitatively 

 and, as far as possible, identified. This necessitates the 

 use of chemical, physical and physico-chemical methods of 

 a very varied nature, which, however, differ fundamen- 

 tally in no way from the methods employed in the study 

 of aerobic metabolism. Hence they are not discussed 

 here. 



Only one point will be emphasized. Some phases of 

 the anaerobic metabolism of invertebrates are frequently 

 investigated by means of manometric methods in media 

 containing bicarbonate. From the results thus obtained 



