§ VI.] Culture of Bacteria. 6i 



existence of intermediate cases, mentioned above, between ex- 

 treme aerobia and anaerobia. 



PfefTer (32) has further shown that chemical stimuli, exerted 

 by other bodies in a state of solution, may influence cells which 

 have the power of locomotion and organisms of very various 

 kinds, hastening and determining the direction of their move- 

 ment, and that the Bacteria supply special instances of this 

 general phenomenon. The chemical bodies which have this 

 effect on the Bacteria are those which were spoken of before as 

 their nutrient substances. The direction of the movement is 

 due, as Pfeffer shows, to diffusion-currents by the introduction of 

 the solutions on one side, the axis of rotation of the cells being in 

 the same direction as the currents and the movement in space 

 in the opposite direction. Other conditions remaining the same 

 the effect varies according to the quality of the body in solution 

 and the concentration of the solution, and it must be particularly 

 observed that it is not every diffusion-current that influences the 

 direction of movement, but only the current from solutions 

 determined in each case by the species of Bacterium. These 

 facts explain a phenomenon which has been frequently observed, 

 namely, that swarms of Bacteria assemble in water round solid 

 bodies, such as dead parts of plants, pieces of flesh and the like, 

 which gradually give off soluble nutrient substances. 



The practical application of these remarks on the conditions 

 and phenomena of vegetation in conjunction with the ascertained 

 facts respecting germs and their dissemination are in the main 

 obvious, if the important points and conditions in each case are 

 kept clearly in mind. We require always a certain amount of 

 positive knowledge and careful consideration of the object which 

 we desire to attain and can really attain in a particular way. The 

 practical remarks therefore may, for the present, be summed up 

 in a very few words. 



First, with respect to the culture of Bacteria, there is but 

 little to be said. Pure extracts of animal and plant-substances, 

 the meat-extracts sold in the shops, broths, the juice of 



