568 Bidler,—Contributions to our Knowledge of the 
Table IV) the attraction of the C 4 H 4 Or ions equals the re¬ 
pulsion of the H + ions. At concentrations below this, owing 
to the specific reaction of the spermatozoa, the combined attrac¬ 
tive effect of the C 4 H 4 0 ^ ions is greater than the combined 
repulsive effect (if such there is) of the H + ions. The 
spermatozoa, therefore, collect in weak solutions of the acid. 
At concentrations above 0*03 °/ o the combined repulsive effect 
of the H + ions becomes greater than the combined attractive 
effect of the C 4 H 4 0 r ions. A repulsion therefore takes place 1 . 
Engelmann 2 discovered that Bacteria are attracted by 
oxygen dissolved in water at a certain pressure and repelled 
when the pressure exceeds a certain limit. In this case the 
same molecules evidently give a positive or negative chemo- 
tactic stimulus according to their concentration. 
Rubidium chloride attracts. If my argument that the Cl~ 
ion is inactive is correct, it is very probable that the Rb + ions 
are responsible for the attraction. The ions Rb + and K + 
attract, while the ions Na + , NH+ and Li + do not. The 
grouping of these elements agrees with that given in the 
Periodic Law. 
It is now necessary to make some remarks upon the 
nature of an ‘ attraction.’ Owing to the fact that the papers 
of Jennings (loc. cit.) came to my notice when too late, 
I made no special study of the motor reactions of the sperma¬ 
tozoa, but assumed that an attraction is due to a sensitive¬ 
ness and consequent reaction on the part of the organisms 
to a gradation in concentration along the surface or in the 
substance of their bodies. If this theory is correct an 
attraction is directly caused by a solution in which the 
spermatozoa collect. There is but one external stimulus. It 
is this, which my application of the dissociation theory may 
concern. The stimulus just suggested is, however, according 
to the observations of Jennings 3 , not the cause of chemotactic 
1 This explanation may be somewhat more complicated, but not different in 
nature, owing to the fact that malic acid dissociates in stages. 
2 Engelmann, Zur Biologie der Schizomyceten. Pfliiger’s Archiv f. Physiologie, 
1881, Bd. xxvi, p. 541. 
3 Jennings, Studies II and V, loc. cit. 
