64 SELECTION AND PREPARATION OF ORGANISMS 



into two groups: first, those features that are absolutely essential, and, 

 second, those features that are desirable but do not make the experiment 

 impossible if these features are not present. Most of the essential features 

 are probably quite obvious, but even these occasionally are forgotten. 

 Forgetting to consider these features in advance might lead one to a 

 rather embarrassing situation. 



Essential features of experimental organisms 



Compatibility With the Prohlem: If some special activity of living 

 organisms is to be studied, then it is important to be sure that this special 

 activity occurs in the organism. Often the problem or hypothesis to be 

 investigated was suggested by previous work with some organism, and 

 it is only natural to proceed with the same animal or plant. Especially 

 when one is beginning in a new area, however, or with a new kind of 

 organism, it is well to perform some preliminary experiments to make 

 sure that the process one is interested in actually occurs in the organism 

 chosen. 



Coynpatihility With the Techniques of Investigation: Most experi- 

 mental research employs chemical methods or physical instruments. Is 

 it possible to use the available methods or instruments upon the organ- 

 ism? A negative answer to this question might be obviated in either of 

 two ways: choose a different technique, or select a different organism. 

 Sometimes an animal or plant is too large or too small. Often special 

 structural features of the cells or higher organizational units make a par- 

 ticular organism impractical. If one kind of plant has cell walls so thick 

 that it is difficult to release the cell contents, perhaps another species 

 should be sought. If the cells of one animal contain so much of one 

 amino acid that it interferes with the analysis of other amino acids, try 

 another animal. 



Availability: As a rule, organisms to be used in experiments should 

 be easily obtainable. This may involve collecting the animals or plants 

 in the vicinity of the laboratory. If the species being investigated is a 

 rare one, the research may be seriously hindered. Difficulties along this 

 line are being experienced in some of the marine laboratories. Extended 

 studies on squid nerve cells have reduced the supply of squids in the 

 vicinity of the Marine Biological Laboratory at Woods Hole, Massachu- 

 setts. The worm Eurekas, a longtime favorite at west coast laboratories, 

 is difficult to find where it used to be plentiful. One means of avoiding 



