60 Transactions Tennessee Academy of Science. 



Substances Eaten. Substances Refused. 



Lactalbumin. Iron. 



Ovalbumin. (**?) Lecithin. 



Peptone. Milk Sugar. 



Tyrosin. Silicic Acid. 



*Uric Acid. Sodium Chloride. 



Uranium Glass. 



Zein. 



This table is of interest in that each column includes some sub- 

 stances which, from the viewpoint of value as food, should be in 

 the other column than the one in which they occur. These are 

 exceptions, it is true, to the statement that ameba discriminates 

 in food, but it must be remembered that physiologists have not 

 yet been able to define the word food from the viewpoint of sense 

 perception, and we can hardly expect ameba to do better than we 

 can. To show that an animal can discriminate it is not necessary 

 that absolutely no "mistakes" be made ; the food selective mechan- 

 ism varies in the ease with which it may be set into operation, and 

 this variable must be taken into account. But the most interest- 

 ing- feature of the table, as has been said, are the exceptions, for 

 in these, doubtless, lies the explanation of the method by which 

 selection is made. Experiments on this point are still in progress, 

 for no definite conclusions concerning the method of selection 

 have yet been arrived at. 



But I have made some observations that may have some bear- 

 ing on the basis of selection of food particles, such as grains of 

 protein, carmine, and the like. I have found that by mechanically 

 agitating particles of the proper size with very fine glass needles, 

 amebas could be induced to eat the particles. All sorts of par- 

 ticles, whether digestible or not, are eaten if properly agitated. 

 Not only is it unnecessary for the agitated particle to lie in con- 

 tact with the ameba, but vibrations produced by the tip of a 

 needle are likewise reacted to. ]:)ositivc]y if the needle i)()int is one- 

 fiftieth of a millimeter or more from the ameba. We mav be 

 certain, therefore, that water vibrations proceeding from a defi- 

 nitely localized source are an efficient cause for successful feed- 

 ing. If the vibrations arising from a non-toxic particle are of 



*Valuo!oss as forxi ; ■-'^*()f fcx! value. 



