METABOLIC GRADIENTS IN AMOEBA 59 



For these reasons, the organic gradients are generally desig- 

 nated as metabolic gradients, although in most cases what we 

 really determine experimentally is the relative susceptibility of 

 various parts of the organism to substances which are toxic or 

 which are used in toxic concentrations. 



As the previous work had been done on axiate organisms it 

 seemed to Professor Child and myself that it would be interesting 

 to try susceptibility experiments on an organism such as amoeba 

 which as no permanent axis. I have used two strains of large 

 amoebae, one grown on hay culture, the other on wheat. 6 Both 

 of these strains are of the granular type and both seem to corre- 

 spond to the description of Amoeba proteus although they are 

 distinctly different in appearance and behavior. I shall have 

 occasion to refer to these differences later. Most of the sus- 

 ceptibility experiments were performed on the hay variety. 



II. SUSCEPTIBILITY EXPERIMENTS 



Potassium cyanide was the reagent employed. It was, of 

 course, essential that the pseudopodia should remain extended 

 during the action of the cyanide if any differential susceptibility 

 was to be detected. This at first seemed unattainable as the 



6 Hay culture: prepared according to the method of Parker ('15). After the 

 culture has stood for one week, a small amount of stale white bread is added, 

 and the culture is inoculated with the amoebae. The culture is successful only 

 when a brown scum appears in it. Wheat culture: boil some wheat for five 

 minutes in a small quantity of water, and put the boiled wheat into jars of water 

 in the proportions of not more than one gram of wheat (dry weight) to a liter of 

 water. Inoculate with unicellular green algae and with amoeba. The amoebae 

 may appear abundantly in a week or two after starting the culture but soon dis- 

 appear and a permanent and rich culture of amoebae is obtained only after the 

 green algae have established a luxuriant growth, about one month from the 

 start of the culture. The amoebae are always on the bottom of such a culture, 

 and continue in abundance for a long time. The life of the culture may be almost 

 indefinitely prolonged by removing some of the water and adding fresh water 

 and a little fresh boiled wheat from time to time. Both the varieties of amoeba 

 were obtained from the Des Plaines river, an old and muddy stream, which 

 receives considerable sewage, near Lyons, 111. I have now abandoned the hay 

 method of culture, and raise both kinds on wheat. This shows that the differences 

 between them are not due to differences in the culture media, since both occur 

 simultaneously in the same jar, although the optimum conditions are not quite 

 the same for the two. 



