LOCOMOTORY DIRECTIVE MOVEMENTS 545 



would have been for all that no need for us to enter into any further discussion 

 of chemotaxis in such organisms, if ROTHERT (1901) had not recently drawn 

 attention to a feature which had escaped previous observers. 



Observation of the slow movements of large Bacteria (such as B. solmsii) in 

 the vicinity of a capillary tube filled with meat extract shows that although the 

 Bacteria pggregate round it, they exhibit no alteration in the direction of their 

 movement when they reach the zone of diffusion ; on the contrary, they often pass 

 close to the opening of the tube, maintaining their original direction of move- 

 ment, and not being apparently stimulated to enter it. At a certain distance from 

 the mouth of the tube they suddenly stop and swim backwards (posterior end 

 foremost). Once more they pass the opening of the tube, unaffected by its 

 contents, and again halt at the same distance from it as in the first instance, 

 and again proceed to carry out a forward movement. They are to be met with 

 in a definite zone opposite the centre of the opening of the tube, but their pre- 

 sence there is obviously purely accidental, and the effect of the stimulus is not 

 one of attraction but of repulsion, induced by the transition to lower concentra- 

 tions, the response consisting, not, as in fern antherozoids, in a reversal of the 

 anterior end, and in a consequent change in the direction of the movement, but in 

 locomotion backwards. After more careful investigation, ROTHERT found that 

 there were greater differences between these two kinds of response than might 

 at first sight have been imagined, for they have only one feature in common, 

 viz. the nature of the stimulant (a chemical compound), while not only the 

 response but also the motive cause of the stimulus and the sensitivity as well is 

 different. It is advisable, therefore, that these two movements should be known 

 by different names, and hence ROTHERT describes the movement where the 

 body of the organism is inverted as strophic chemotaxis, and that where the 

 organism swims backwards as apobatic chemotaxis. 



Wherein then lies the difference in the motive cause of the stimulus in 

 these two series of phenomena ? PFEFFER and ROTHERT look upon the strophic 

 chemotaxis as due to differential distribution of the stimulant on different sides 

 of the plant, as in the case of chemotropism, heliotropism, &c. ; in other words, 

 the organism measures and compares the intensity of the stimulant as effecting 

 different parts of its outer surface. It is impossible, however, that a fern 

 antherozoid can appreciate the inequality of the stimulus on opposite sides, 

 since, owing to its rotation on its own axis, any unilateral influence of the 

 stimulant is excluded, just as when a higher plant is rotated on a klinostat. 

 (This criticism, which has not as yet been published, has been communicated 

 to me by OLTMANNS.) The organism must also be able to compare the inten- 

 sity of the stimulant at its anterior and posterior ends, and, on the analogy 

 of a dorsiventral body, must not be in a state of equilibrium, when both ends 

 are subjected to the same intensity of stimulus, but only when the intensity of 

 the stimulus is greater at the anterior end ; this will be the case at least as 

 long as positive chemo tactic movement follows, in negative chemotaxis the 

 inverse relations hold good. 



In apobatic chemotaxis it is possible that the motive cause of the stimulus 

 lies at least in differential concentration of the stimulant at the two poles of the 

 organism, but it is more probable that we have here to do rather with differences 

 in time than differences in place, and that response occurs when the organism 

 has remained for a definite length of time in a solution of the stimulant less 

 concentrated than that in which it was a short time previously. A homo- 

 geneous solution, therefore, must in this case also act as a stimulus, and the 

 bacterium will move backwards if it be transferred from a 10 per cent, solution 

 of meat extract to one of 5 per cent. JENNINGS (1897 and onwards) has, as a 

 matter of fact, shown this to be the case in motile Infusoria (Paramoecium), but 

 Bacteria present experimental difficulties too serious for accurate investigation. 

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