STIMULI AND THEIR ACTIONS 



445 



that the plasmodia slowly crept from the tan upward in the strip 

 over the edge of the beaker arid downward upon the inner side of 

 the glass, until they spread themselves out upon the surface of 

 the water. By proper control-experiments it was possible to 

 determine with certainty that it was only the streaming water 

 which afforded the stimulus. 



Unfortunately the rheotactic properties of other organisms have 

 been little investigated. It is, however, very probable that 

 rheotaxis is wide-spread. Among other cases, it is easy to assume 

 that the human spermatozoa are rheotactic and find their way to 

 the egg-cell by means of this property. When the spermatozoa 

 come into the uterus, they meet a current of mucous liquid coming 

 toward them, since the cilia of the epithelium lining the uterine 



FIG. 220. Thigmotaxis of Paramcecium. A, An individual in contact with a fibre of filter-paper ; 

 the cilia that touch the fibre directly are still. , Assemblage of Paramacia about a bit of 

 filter-paper under the cover-glass. (After Jennings.) 



cavity have a direction of stroke toward the os, and hence produce 

 a current toward the outside. That it is chemotaxis of the 

 spermatozoa toward the ovum which points out the path to them 

 becomes very improbable when it is remembered ^that the sperma- 

 tozoa wander upward in the uterus before the ovum has left the 

 ovarian follicle. As a matter of fact Roth ('93) has succeeded in 

 showing experimentally that spermatozoa and likewise certain 

 Bacteria are rheotactic, by producing under the cover-glass a feeble 

 continuous current and observing that these unicellular organisms 

 move in opposition to it. 



As a third form of barotaxis we have to consider finally geotaxis, 

 i.e., the phenomenon that certain organisms place themselves and 

 move with their median axis in a very definite direction toward 



