GROWTH AND MOVEMENT 447 
Chemotaxy. Chemotaxy has been most extensively investigated, 
but is not yet fully elucidated. If a soluble crystal be introduced into 
water undisturbed by currents, the molecules gradually diffuse from 
its surface in a constantly enlarging sphere; or if the water be the film 
under a cover glass, in an increasing zone. By using a glass tube drawn 
out to a very fine capillary and closed at one end, liquids of any sort 
may be used. A short capillary is filled with the solution and placed 
on a microscope slide with its open end under the cover glass. Slow 
diffusion takes place from the mouth, while the behavior of the organisms 
is watched under the microscope. As a rule the rate of their movement 
is not affected, except by substances that are directly injurious. Tt 
appears that the directive effect of such stimuli is exercised in two dif- 
ferent ways. 
1 . Orienting reaction. In the first case, the direction is altered 
because the organism, in response to the stimulation, orients itself, so 
that with continued movement the body will be carried toward or away 
from the source of the diffusing molecules. It is assumed that this 
orientation is determined by the unequal or one-sided action of the 
molecules, the end (less probably the flank) toward the source being most 
powerfully affected, whereupon the creature turns, and according as it 
brings the anterior or the posterior end toward the source of stimulus, 
and swims, it will approach or recede from that source. 
2. Recoil reaction. The second case is quite different. The move- 
ments of sperms and zoospores are too rapid to be followed easily; but 
if large and slow-moving organisms are observed, they may be seen to 
swim about quite indifferently, passing in close proximity to the crys- 
tal or capillary tube from which the molecules are diffusing, without 
showing any tendency to swim towards it. But when they reach by 
chance the limits of the diffusion zone, they suddenly reverse their direc- 
tion and back away, as though they had encountered an obstacle and 
had rebounded from it. This reaction is repeated at every side, and 
having once chanced to swim into the diffusion zone, they are imprisoned 
within it, because the attempt to pass out of it results always in the re- 
action of recoil. So, as more and more are thus caught, there is an 
accumulation within the diffusion zone, as though it were a trap. Not 
all substances, however, permit the first accidental entry, for the recoil 
may be produced at the attempt to enter this zone, while any such organ- 
isms placed within it would be free to swim out without recoil. In such 
a case the final result is the accumulation of the organisms in the regions 
