238 J. GRAHAM EDWARDS. 



contractile vacuole in appearance (Figs, i, 2, 3 and 4). This 

 vesicle is then constricted off from the ectoplasm and borne along 

 in the endoplasmic current as a normally formed food-vacuole. 

 The size of the concavity varies depending apparently on the size 

 of the pseudopod giving rise to it. If the amoeba is monopodal, 

 the concavity and vesicle are large; if heteropodal, they are 

 smaller. The upper as well as the lower surface may develop 

 such a concavity and vesicle so that contact with the substratum 

 is not necessary. The food-cups thus formed rarely remain 

 visible for longer than 3 to 5 minutes. 



The most favorable media for inducing these feeding reactions 

 are as follows: A mixture of NaCl, LiCl and CaCl 2 ; NaCl alone 

 or mixtures of neutral sodium salts; CaClo and a mixture of 

 NaCl with CaCl 2 ; or NaNO 3 mixed with Ca(NO 3 ) 2 . Mixtures of 

 the different salts used were always of the same concentration and 

 in equal parts. Suitable concentrations are N/^oo or N/$oo. No 

 appreciable difference exists in either concentration as to the size 

 or number of food-cups formed. Sea-water diluted by adding 

 two parts of distilled water to one of sea water is also an excellent 

 medium for such feeding reactions. Less favorable media are 

 KC1, A 7 /5OO, and mixtures of potassium salts, MgCl 2 and MgSO 2 . 

 In neutral ammonium salts and in SrCl 2 and BaCl 2 , N/^oo, these 

 reactions occur only occasionally. 



In the most favorable mixture, i.e., NaCl, LiCl and CaCl 2 , 

 containing a hundred or more amoebae, this reaction is observable 

 within one or more hours in a considerable number of specimens 

 and thereafter almost continuously in now one group, now 

 another. In a few hours it is possible to obtain as many amoebae 

 as desired in all of the stages manifest in this reaction. Owing to 

 the rigidity of the marginal surface of the food-cup, amcebas can 

 be removed from the solution with a pipette and immersed in a 

 fixing fluid without appreciable change in the character or form 

 of the food-cup. Examination of food-cups when amoebae are in 

 the solution as well as after they are fixed and sectioned fails to 

 show the presence of any particle or other visible stimulus 

 normally present in feeding reactions. 



Feeding reactions also occur in a dilute acid or base, e.g., HC1 

 or NaOH if a neutral salt, e.g., NaCl is added. Feeding reactions 

 were not observed to occur in other than neutral salt solutions 



