298 THE PROTOZOA 



Oxygen, as with most organisms, exercises a chemotactic effect, 

 although the positive are much more rare than negative effects 

 (Verworn and Jennings). Jennings, in a series of very careful ex- 

 periments, found that alkalies give clearly marked negative chemo- 

 taxis, while substances "in which Paramecia collect, giving the 

 motor reaction only when they attempt to pass out of them, are 

 substances having a weak acid reaction " ('99). The relative effect 

 of the same stimulus upon different organisms is prettily shown 

 by Massart's ('91) experiments upon Spirilhcm and Anophrys, in the 

 presence of oxygen. Both organisms collect in a ring around a bubble 

 of air, the ciliate nearer the source, the bacillus away from it. Carbon 

 dioxid is said to give the same reaction as dilute acids, causing the 

 aggregation of Paramecium about it (Jennings), although other ob- 

 servers have been unable, thus far, to confirm these experiments. 



The singularly interesting investigations which Pfeffer ('84, '88) 

 made upon the chemotaxis of reproductive elements have thrown no 

 little light upon the problems of fertilization, and have shown at the 

 same time how minute a quantity of the stimulant is needed to give 

 the required effect, too much- causing a negative result. He found 

 that antherozoids of the fern are attracted to the capillary tube, which 

 holds a slight trace of malic acid, while a larger quantity turns them 

 away. Carrying the experiment to other forms of life, it is probable 

 that analogies to this may be found in almost all kinds of organisms, 

 both animal and plant, and that positive chemotaxis is a necessary 

 condition of the fertilization of many eggs by spermatocytes. 



Jennings's ('99) interesting experiments and observations upon 

 various forms of Infusoria and Mastigophora show that many hith- 

 erto supposedly directive chemical stimuli are effective, not because of 

 their unilateral stimulation through the differences in density, but 

 because they induce a natural motor response on the part of the organ- 

 ism. To illustrate with the case of Paramecium, which affords the 

 most satisfactory basis for this view, Jennings maintains that these 

 organisms, which for a long time have been known to have an appar- 

 ent affinity for weak acids, are not attracted by a drop of acid placed 

 in the vessel which contains them, but wander into it in their aimless 

 movements. The usual motor response of Paramecium is to swim 

 backward from the object struck, turn toward its side containing the 

 peristome, and swim forward again. By this manoeuvre an ordinary 

 obstacle is avoided. The acid does not cause this reaction, but when 

 the organism attempts to leave the drop, contact with the surrounding 

 walls brings about the motor response, and the organism thus remains 

 entrapped (Fig. 151). While this ingenious view is acceptable in the 

 case of Paramecium, it by no means follows that all so-called chemo- 

 tropic or chemotactic effects can be similarly explained, and that 



