438 GENERAL PHYSIOLOGY 



Jennings performs his experiments upon the slide under a large 

 cover-glass which is supported by two glass rods, so that a pretty 

 thick layer of water containing Paramceda lies between the slide 

 and the cover. In this layer, which must be free from all admix- 

 tures, he places carefully by means of a pipette drawn out into a 

 capillary, a drop of the solution the chemotactic action of which 

 is to be investigated (Fig. 215). The substances in this solution 

 diffuse at once into the surrounding liquid, in which the Paramceda 

 are scattered uniformly and in motion. Then, according to the 

 mode of action of the substances in question, very characteristic 

 effects are produced. If the substances are ineffective, as, e.g., solu- 

 tions of sugar, the Paramceda swim undisturbed into the drop, and 

 after a few seconds are again spread uniformly under the cover-glass. 

 If the drop induces negatively chemotactic properties, as, e.g., 

 alkalies, a circle that is completely free from Paramceda, forms at 

 the place (Fig. 215, A). But if the drop induces positive chemo- 

 taxis, as e.g., most acids, all Paramceda that are present under the 

 cover-glass swim into it (Fig. 215, B\ If the concentration of the 

 effective substance is above the optimum, the Infusoria accumulate 

 about the drop in a circular zone (Fig. 215, C\ It is remarkable 

 that Paramceda are positively chemotactic toward carbonic acid, 

 as toward other acids. If a bubble of chemically pure carbonic acid 

 and at the same time, for control, one of ordinary air be placed 

 under the cover-glass, the Paramceda, leaving the air, congregate 

 in a dense mass about the carbonic acid (Fig. 215, D). But, in 

 proportion as the latter diffuses into the water and accumulates in 

 a concentration above the optimum, they retreat in a closed circle 

 from the bubble, because they are negatively chemotactic toward 

 strong solutions of carbonic acid. Thus very characteristic figures 

 appear (Fig. 215, JE). Further, since Paramceda, like all other 

 organisms, produce carbonic acid, constantly more individuals 

 become attracted to the place where for any reason a number have 

 assembled. We have here a very interesting case of the formation 

 of an assemblage simply by reason of positive chemotaxis. As 

 Fig. 215, B shows, by the transference of a drop of water from 

 a group to another cover-glass preparation containing Paramceda 

 a chemotactic assemblage may be produced. 



Finally, chemotactic phenomena afford a means of forming an 

 approximate conception of how extremely slight may be the stimuli 

 that are able to exert a visible effect upon living substance. In 

 his experiments Pfeffer found that the spermatozoids of ferns 

 exhibited distinct chemotaxis when he employed a capillary tube 

 containing a solution of O'OOl per cent, malic acid. If it be borne 

 in mind that the malic acid must first diffuse into the drop in 

 order to exercise its power, it follows that the quantity 

 that acts upon the spermatozoids must be exceedingly small, 

 b ut this is not all. In order to produce a chemotactic effect it is 





