40 CHEMICAL AGENTS AND PROTOPLASM [Ch. I 



that malic acid attracts. The strengths of solutions which 

 attract under such conditions lie between 0.001% and 10%. 

 The weaker solution may be designated the minimum ; the 

 stronger, the maximum concentration which provokes a re- 

 sponse. The minimum solution provoking response is also 

 often called by the Germans the " Reizschwelle," or "stimula- 

 tion threshold";* the optimum, the "Reizhohe," or "stimu- 

 lation acme"; the range, the "Reizumfang." 



The character of the responses observable at the two limits 

 is very different ; at the minimum, attraction is very feeble ; 

 thus, while a capillary tube containing 0.01% neutral sodic 

 malate, plunged into water at 14:°-20° C, swarming with 

 spermatozoids, attracts 400 of them in 10 minutes, a 0.001% 

 solution attracts only 10-25 individuals during the same time, 

 and a 0.0008% exerts little attractive effect, the spermatozoids 

 remaining undirected in their movements. At the maximum, 

 on the contrary, repulsion is observed. The spermatozoids 

 move from the mouth of the capillary tube. Between the 

 two extremes lies the concentration of greatest attraction — 

 the acme. As we pass from the acme towards the minimum, 

 the attraction becomes less and less. As we pass towards the 

 maximum, the attraction remains the same, or increases ; but 

 repelling influences are now at work, which eventually entirely 

 counteract the attractive influences. 



A satisfactory method of expressing quantitatively the facts 

 just mentioned has not been invented. Pfeffer ('88, p. 599) 

 has employed the nomenclature which we have used above 

 (p. 36) — flj to (Zg being combined with r^ to r^ to indicate 

 the coworking in varying proportions of attraction and repul- 

 sion. Using this nomenclature, we may illustrate the state- 

 ments made in the last paragraph with examples taken from 

 Pfeffer's work : — 



* The following substances at the solutions named produce the threshold 

 attraction (ai) in Bodo saltans: KCl, 0.02%; K3PO4, 0.002%; KH2P()4, 

 0.0035%; KNO3, 0.26%; K2SO4, 0.22%; KCIO3, 0.3%; K4(CN)6Fe, 0.285%; 

 Ka-C^HiOe, 0.02%; RbCl, 0.14%; LiCl, 0.6%; LiNOs, 3%; NH4CI, 0.3%; 

 neutral ammonium phosphate, 0.08%; SrCla, 0.2%; Sr(N03)2, 0.4%; BaCla, 

 0.17%; dextrin, 0.1%; urea, 1%; asparagin, 0.1%; taurin, 1%; sarkin, 

 0.33%; pepton, 0.01 0/,; meat extract, 0.01%. 



