STIMULI AND THEIR ACTIONS 435 



freely-moving sperm-cell (Fig. 212). The fact, which must other- 

 wise appear very wonderful, that among the innumerable swarm of 

 spermatozoa of the various, marine animals every species finds 

 the proper ovum, is in a great majority of cases a direct result 

 of chemotaxis, and is explained very simply by the further fact 

 that every species of spermatozoon is chemotactic to the 

 specific substances that characterise the ovum of the corres- 

 ponding species. We have here an adaptive phenomenon of the 

 simplest kind, which gives us anew an idea of how extraordinarily 

 deeply the phenomena of chemotaxis reach into life-relations. 



Pfeffer's experiment was as follows : He filled a capillary tube, 

 sealed at one end, with a solution of c. 0'05 per cent, malic acid 

 and placed it in a drop that contained a great number of the 

 spermatozoids of a fern ; the malic acid gradually diffused from 

 the opening of the tube into the drop, and thus became the source 



FIG. 212. Two ova of the plant, with spermatozoids swarming about them. (After Strasburger.) 



of a stimulus acting unilaterally. Microscopic examination showed 

 that the spermatozoids immediately began to steer toward the 

 opening of the tube and to swim into it. After a half-minute 

 some 60 spermatozoids had entered the capillary, and in some 

 cases after five minutes approximately 600. In one experiment 

 with 24 spermatozoids, after twelve minutes all except one, which 

 lay at rest outside, had collected in the capillary. Hence malic 

 acid exerts a very strong chemotactic action upon the sperma- 

 tozoids of ferns ; toward all the other substances that Pfeffer 

 tested with respect to their chemotactic power, the spermatozoids 

 behaved with complete indifference. This suggested strongly 

 the supposition that in the archegonium that holds the ovum it is 

 malic acid that causes the spermatozoids to approach and enter. 

 On account of the minuteness of the objects and the lack of 

 micro-chemical methods, Pfeffer was not able to demonstrate 

 malic acid in the contents of the archegonia themselves, but 



F F 2 



