2 2 (86) Society for Experimental Biology and Medicine. 



ferral of the animals from one vessel to another, but this has not 

 yet been ascertained with certainty. Half an hour or an hour 

 later, the negative gathering of the animals becomes less dense, 

 and the animals are scattered in the vessel. 



These negatively heliotropic animals can be transformed in- 

 stantly into positively heliotropic animals by the following sub- 

 stances : (i) many of the anesthetics of the fatty series ; (2) many 

 acids, except very weak ones like boric acid ; (3) certain salts, 

 like ammonium salts. Alkalis, like NaOH or Ba (OH) 2 , and 

 neutral salts, like NaCl or CaCl 2 , bring about a scattering of the 

 negatively heliotropic animals, but do not make these animals 

 instantly and without exception positively heliotropic, as do the 

 anesthetics or the acids. 



A few quantitative data may be mentioned here by way of 

 illustration. Ethyl acetate and similar esters instantly make all 

 the shrimps positively heliotropic in a concentration of about 

 Ether is effective at a higher concentration, namely, ~. Ethyl 

 alcohol brings about an equally rapid transformation of the 

 negatively heliotropic animals into positively heliotropic ones in a 

 much higher concentration, namely, 2\ m solution. Paralde- 

 hyde is active at a concentration of about -f\. So far as the 

 acids are concerned, HC1, oxalic acid and acetic acid make the 

 animals instantly and without exception positively heliotropic in a 

 concentration of about -g^. In boric acid, the animals remain 

 negative even at a concentration as high as y 1 -. C0 2 acts in the 

 same way and, inasmuch as this substance is produced in the ani- 

 mal itself, and as the amount produced varies under certain con- 

 ditions, we may now be able to account for apparently " spon- 

 taneous " changes in the heliotropic sensitiveness of these and 

 other animals. NH 4 C1 instantly makes the negatively heliotropic 

 Gammarns positively heliotropic in a concentration of NH 4 OH 

 acts similarly in the same concentration. So far as neutral salts 

 are concerned, they usually do not make the negatively heliotropic 

 Gammarns positively heliotropic until the concentration is over '| 

 or 5, and even then, as a rule, no instantaneous and complete 

 gathering of the animals at the positive side of the vessel can be 

 produced, but only a slow and partial gathering. 



Since the concentration at which the transformation of nega- 



