Physiology of the Spermatozoa of Ferns . 569 
movements in the Flagellata and Ciliata. A collection of 
these organisms in a drop introduced into the medium where 
they are swimming takes place as follows. (1) The organ¬ 
isms enter the drop accidentally, (2) the drop acts upon the 
organisms, (3) in consequence, when they accidentally approach 
their previous medium it repels them. Here there are two 
distinct kinds of action. The drop is responsible for the 
first, and the previous outer medium for the second. If 
the motor reactions of the spermatozoa are of the same 
nature as those of the Ciliata and Flagellata it must be 
understood that when, in the application of the dissociation 
theory, ions or molecules are said to ‘attract/ this means 
that they give rise to the first of the two actions. 
In conclusion it must be pointed out that in discussing 
the attraction of neutral salts I have been unable to state 
whether the whole molecules play any chemotactic rdle. 
The omission is, however, rendered less serious by two con¬ 
siderations. Firstly, owing to the high dissociation at the 
low concentrations used, the number of whole molecules 
in any unit volume of a solution is usually very much less 
than the number of ions. Secondly, the ions in any solution 
are supposed by chemists 1 to be responsible for its chemical 
reactions. The assumption that the attraction by a neutral 
salt is practically due to one or both ions is therefore to 
a considerable extent justified. Since, however, in the work 
upon toxicity (loc. cit.) it has been shown that whole mole¬ 
cules in a dissociated solution are poisonous and therefore 
have a physiological action, no application of the dissociation 
theory to chemotaxis can be considered complete until it 
has been determined what effect, if any, is due to the whole 
molecules. 
General Points. If it is desired to determine whether 
a neutral salt attracts or not, experience teaches us that this 
may best be attempted by trial at a concentration of about 
tV m °h 
1 E. g. Ostwald, Grundriss der Allgemeinen Chemie, i 8tj Aufl., 1889; also 
Whetham, Solution and Electrolysis, Cambridge, 1895, p. 165. 
