554 Butter.—Contributions to our Knowledge of the 
the concentration of the solution has so far sunk that attraction 
no longer takes place, they may move out again. In several 
cases I could detect a weak attraction only after repeating 
the experiments a considerable number of times. 
Malic acid and its neutral salts have a strong attraction 
(denoted by A) at a concentration isotonic with T oVo mol 
potassium nitrate. There is no hesitation in entering the 
tubes, where the spermatozoa continue to swim for some 
minutes. In a successful experiment several hundred organ¬ 
isms may be collected. 
When a solution acts indifferently the spermatozoa enter 
the tube occasionally, but do not collect there. If uninjured 
by the solution they move as readily out of the tube as 
into it. Indifference may be well observed when the tube 
contains tap-water. In making the experiments this control 
was often employed. 
A repulsion denoted by R is always very definite. The 
substances (two acids and an acid salt) found to give such 
a repulsion also attract at lower concentrations. If a tube 
contains one of these repellent acid substances at sufficient 
concentration the spermatozoa are attracted towards the 
mouth. On reaching a diffusion-zone which causes repulsion 
the spermatozoa reverse their direction of motion and move 
rapidly away from the mouth of the tube in a fairly straight 
line, and thus reach a zone which no longer repels. Here 
the normal more or less circular direction of motion is 
resumed. The flight from a repellent zone is easily re¬ 
cognized. Owing to the attraction at low concentrations 
and repulsion at higher the spermatozoa may collect in a 
ring outside the tube. The ring indicates the position of 
a diffusion-zone at the intermediate concentration. During 
an experiment, owing to diffusion, the zone gradually 
approaches the mouth of the tube which it may finally 
enter. 
It must be pointed out that the repulsion R just described 
differs considerably from the repulsion which makes itself 
evident in the case of a marked attraction (A) by a neutral 
