CHEMICAL ATTRACTION 109 



tartaric acid, acetic acid, and other vegetable products, 

 without success the sperms took no notice. At last (I 

 believe it was the seventeenth of the substances he tried) 

 he filled one of his little tubes with a dilute solution of 

 the acid which is found in pears and apples, and is called 

 " malic acid " (from the Latin " malum," an apple) and 

 placed it as before. A marvellous sight then greeted his 

 eye. He saw the little sperms violently and eagerly 

 swimming to the minute glass tube containing the malic 

 acid, and crowding into it. This, then, was the attractive 

 substance produced by the egg-pit of the fern's prothallus ! 

 The microscopic, screw-like particles of protoplasm are 

 guided in their movement by the dilute stream of malic 

 acid issuing from the egg-pit. It is a curious coincidence 

 that the name of this acid is appropriate as being irre- 

 sistibly attractive to the male fertilising sperms. 



Pfeffer gave to this process of the chemical guidance 

 of simple protoplasm which is in its results similar to 

 that produced by the sense of smell and taste in higher 

 organisms the name " chemotaxis." It has since been 

 recognised as a general process of great importance in 

 affecting the movements of minute protoplasmic particles 

 such as the white or colourless corpuscles of the blood 

 (phagocytes) and the ubiquitous swarming bacteria and 

 bacilli. Chemotaxis may cause attraction, and is then 

 called " positive chemotaxis," or it may cause repulsion, 

 when it is called "negative chemotaxis." The chemical sub- 

 stances which produce it are by no means limited to malic 

 acid, but are endless in variety : oxygen gas, as well as 

 various elaborate organic substances, are " positively 

 chemotactic " to many microscopic organisms ; weak acids 

 and such bodies as quinine are negatively chemotactic. 

 The moving towards and repulsion from other sources of 

 " stimulation " are conveniently spoken of by the use of 

 the same words, " taxis " and " tactic " ; thus the attraction 



