74 



PHYSIOLOGY 



CHAP. 



generally observed was contraction, i.e. retraction of the pseudo- 

 podia. The most various chemical substances are capable of 

 producing this effect : 1-2 per cent solution of sodium chloride, 

 dilute hydrochloric acid O'l per cent, caustic potash 1 per cent, 

 weak solutions of other acids, alkalies, or salts. 



The Ehizopods treated with these solutions assume a globular 

 form on retracting their pseudopodia, owing to the concentric 

 contraction of the protoplasm (Fig. 18). Ciliated cells, on the 

 contrary, when treated with the same stimuli, increase their 

 vibratile movements sometimes to a very marked extent. Smooth 

 and striated muscles contract, and sometimes exhibit a rhythm of 

 contraction that they do not normally possess, recalling the 

 rhythmical movements of the vibratile cilia. 



Besides the contractile effects of chemical stimulation, it is 



FHJ. is. Actinosphaerimn Eichhornii. (Venvorn.) a, under normal conditions ; h, at commence^ 

 ment of chemical excitation, the filiform pseudopodia are contracted and varicose ; <;, after 

 prolonged chemical excitation, the pseudopodia are completely retracted. 



possible also to observe expansive effects, i.e. active elongation of 

 the pseudopodia in Amoebae, Myxomycetes, etc., effects which in the 

 first instance were studied by Kiihne. On placing an amoeba, for 

 instance, in a gas chamber in which oxygen has been substituted 

 for hydrogen, the movements are suspended after a short time. 

 On again admitting oxygen, the amoebae, after twenty-four hours 

 of inactivity, at once begin to expand their pseudopodia with 

 normal vivacity. 



Even more important than this direct excitation are the 

 phenomena of the directive action of chemical stimuli upon the 

 movements of elementary organisms, phenomena known as chemo- 

 tactic or chemotropic. Chemotaxis, as first discovered by Engelniann 

 on Bacteria, observed by Stahl on Myxomycetes, and studied on a 

 large scale by the botanist Pfeffer in 1887, has assumed a great 

 importance. 



Positive must be distinguished from negative chemotaxis. The 



