170 W. C. ALLEE 



with N/10 solutions of the other cations. Even with this dif- 

 ference sodium is least toxic, followed after a long interval by hth- 

 ium, caesium, rubidium and potassium in the order named. 

 The favorable effect of sodium chloride is shown by the fact 

 that ten isopods were reacting after eleven hours and at fourteen 

 hours six of these were more positive than at the beginning. 

 Three were still reacting after sixteen hours and lived for five 

 days in N/5 solution. 



The cation NH4 is chemically closely related to the other 

 members of this group and is often similar in physiological action 

 to potassium which it particularly resembles. In its effect on 

 rheotaxis, however, ammonium chloride is less effective than 

 caesium chloride so that the complete series would be NH4 < 

 Cs < Li < Na < Rb < K. The toxicity effect of ammonium 

 is hke that of potassium and rubidium as shown by the series 

 Na < Li < Cs < Rb < NH4 < K. This is in the same order 

 as has been found to preserve the irritabihty of fresh frog's 

 nerve (Brodsky '08, vide Hober s. 511); to cause recovery of 

 irritabiUty after loss in cane sugar solution (Hober, s. 497) 

 namely: Na < Li < Cs < NH4 < Rb < K. The only dif- 

 ference is that in the foregoing instance ammonium stands 

 next to rubidium while with the isopods it is next to potassium. 



I am unable to state why ammonium should act in its usual 

 manner as regards toxicity and not as regards rheotaxis. • The 

 rheotactic reaction may be influenced by the well developed 

 nervous system of the isopods or by direct action on the muscles 

 or by a combination of the two. It may be we have here a 

 differential effect in that potassium stimulates the muscles more 

 than it depresses the central nervous system while ammonium 

 acts in the opposite manner. Mathews ('07) found that these 

 two cations have similar effect on muscles and on motor nerves 

 but are opposed in their effect on the central nervous system, 

 which potassium depresses while ammonium stimulates it to 

 the point of tetanic convulsions. The action may also be ex- 

 plained by assuming that ammonium produces a greater per- 

 meabihty than potassium such as Lilhe ('09) discovered with 

 Arenicola larvae. It is known (Mathews '07) that the am- 



