THE PROBLEM AND METHODS OF INVESTIGATION 77 



in the cleavage stages of eggs, and Loeb^ and others have made use 

 of the cyanides to decrease or inhibit the oxidation processes in 

 eggs, and Drzewina and Bohn ('13) have observed parallel differ- 

 ences in susceptibility to cyanides and lack of oxygen along the lon- 

 gitudinal body-axis of certain flatworms. Some other incidental 

 observations also exist, but the general significance of differences 

 in susceptibility has been either ignored or not recognized. 



THE DIRECT METHOD 



By this method the resistance or susceptibility is determined 

 directly by concentrations of cyanide or other agents which kill 

 the animals within a few hours. For a particular species a con- 

 centration must be determined which kills without acclimation, 

 but which does not kill so rapidly that the differences in suscepti- 

 bility do not appear clearly. For Planaria dorotocephala (see p. 93) 

 and other related species a concentration of one one-thousandth 

 gram-molecular solution (o.ooi mol., 65 milligrams per liter, 

 0.0065 per cent) of potassium cyanide has been found most satis- 

 factory at temperatures about 20° C. and for most purposes. This 

 kills the animals in from two to twelve hours according to their con- 

 dition. But a range of concentrations from 0.0002 mol. up to 

 0.005 mol-, or even higher, may be used, except where the meta- 

 bolic rate is very high, as in young animals, without altering any- 

 thing but the time factor. Essentially the same results are obtained 

 from 4 per cent alcohol or from 2 per cent ether as from o.ooi mol. 

 potassium cyanide. 



Since the death and disintegration of different parts of the body 

 usually follow a regular sequence (Child, '136), it is possible to 

 determine the time, not merely of disintegration of the whole ani- 

 mal, but of the various regions of the body. The body of Planaria 

 consists of two or more zooids (see p. 123) of which only the anterior 

 one is morphologically developed. In this anterior zooid death and 

 disintegration usually begin at the head-region and proceed pos- 

 teriorly, and the lateral margins of the body usually die and disin- 

 tegrate before the median region. The most satisfactory method 



'Loeb, '09, '10; Loeb and Lewis, '02; Loeb and Wasteneys, '10; and various 

 other papers. 



