§ 3] ELECTROTAXIS 149 



the fish with their heads to the anode was noticed. With 

 Salamandra larvsB currents of 2.3 8 to 4.7 8 were chiefly em- 

 ployed. In the experiments of Blasius and Schweizbk the 

 organisms sometimes migrated toward the anode, if the cur- 

 rent was not so strong as to stupefy, but they lay stress upon 

 the point that the migration is a secondary phenomenon — that 

 the orientation is the primary effect of the current. 



Next came the observations of Ewald ('94), who used very 

 young tadpoles (5 days) and non-polarizable points as elec- 

 trodes. Thus he was not able to give the strength of current 

 to which the individuals were subjected. His results seemed 

 directly to oppose those ■ of the two preceding authors, for 

 with his (unknown) current, the tadpoles were stimulated 

 when the current passed caudad and stupefied by one passing 

 cephalad ; also they placed themselves in the axis of the cur- 

 rent with their heads towards the kathode. The larvae did 

 not seem to find this position as a direct response to stimulus, 

 but whenever an individual, in its turnings to the right and 

 left, fell into this — electrotactic — position it was no longer 

 stimulated, but stupefied, and so came to rest. 



These discordant results of Ewald led Hermann, with his 

 student Matthias, again into the discussion. By careful 

 measurements of the strength of current, they found that 

 between 0.38 and 1.5 8 frog tadpoles of from 1 to 3 weeks old 

 did face the kathode, as Ewald found, and did move towards 

 it. But Hermann and Matthias ('94) believed this result 

 to be due to the fact that only cephalad-flowing currents of this 

 intensity excite, and thus only such currents are able to pro- 

 duce locomotion. 



Ewald ('94"), however, cannot accept their idea that a 

 caudad-passing current of small intensity produces no excita- 

 tion, for, he says, he has seen small fish, lying with face to the 

 anode, made to move towards the kathode by the action of the 

 weak current. Since Hermann and others have shown that 

 very strong currents cause paralysis even when flowing cepha- 

 lad, Ewald concludes that we must recognize the existence of 

 three different effects at three intensities ; weakest, medium, 

 and strongest. The medium current (which has the broadest 

 range) is that by which the organisms are irritated as it flows 



