556 Butler,—Contributions to our Knowledge of the 
coming to rest, but as a rule are carried out again by- 
currents. 
It has been remarked that when spermatozoa pass down 
a tube containing a neutral salt isotonic with mol po¬ 
tassium nitrate they come to rest within one minute. They 
do not penetrate more than half-way down a tube and usually 
less. With an isotonic solution of glycerine or alcohol the 
spermatozoa which have entered a tube accidentally may 
swim down to the bottom and for some minutes before 
coming to rest. On entering the alcohol the spermatozoa 
continue to swim as rapidly as in water. On entering the 
glycerine, their motion is much less rapid. If a solution of 
glycerine isotonic with i mol potassium nitrate is in a tube, 
spermatozoa accidentally entering penetrate somewhat further 
and move longer than in a similar experiment with the latter 
salt. With an isotonic solution of alcohol (6-9 grams per 
cent.) the contrast with the neutral salts is very striking. On 
entering a tube a spermatozoon may move down to the 
bottom, and for one to five minutes. In a similar experiment 
with potassium nitrate, as already stated, the spermatozoa at 
best only penetrate a distance equal to about one diameter 
and come to rest immediately. 
The explanation of the above differences depends upon the 
rate of penetration of the substances in question through 
living membranes. Neutral salts, sugar, &c., penetrate ex¬ 
tremely slowly or not at all, glycerine 1 comparatively quickly, 
and alcohol 2 very quickly. A spermatozoon comes to rest 
when a certain amount of water is withdrawn from it. It 
seems to follow, therefore, that the distance a spermatozoon 
may penetrate down a tube depends partly upon the specific 
rate of penetration through living membranes of the substance 
tested. 
1 De Vries, Ueber den isoton. Coeff. des Glycerins. Bot. Zeit., 1888, nos. 15 
and 16. 
2 Overton, Ueber die allgemeir.en osmotischen Eigenschaften der Zelle, ihre 
vermntblichen Ursachen nnd ihre Bedeutung fur die Physiologie. Vierteljahrschrift 
der Naturforschenden Gesell. in ZUrich, xliv, 1899. 
