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. It 

 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 



