THE VITAL PROPERTIES OF THE CELL 121 



varying from two to five minutes, a distinct plug of bacteria is 

 seen to have collected at the mouth of a capillary tube, which has 

 been placed in a drop of water containing these micro-organisms. 



On account of the different ways in which various cell bodies 

 react towards different chemical stimuli, the method, which 

 Pfeffer has perfected and used with various reagents, may be 

 employed, not only to attract one individual organism sensitive to 

 one special reagent, but also to separate different species which are 

 mixed together, as has also been done by means of galvanotropism 

 or heliotropism. Glass tubes provided with suitable attractive 

 material, and inserted in fluids, may be used as traps for Bacteria 

 or Infusoria. 



Further, it follows from the above-mentioned experiments, that 

 organisms which are specially sensitive towards a given chemical 

 substance may be used as reagents to indicate the presence of 

 this stimulating substance. Thus, according to Engelmann (IV. 

 7), certain Schizomycetes form an excellent test for oxygen, of 

 which such a minute portion as one trillionth of a milligramme 

 is sufficient to attract them. 



Not every substance which attracts an organism is useful to it 

 as food, or is even innocuous to it ; many, such as sodium salicylate, 

 saltpetre, strychnine, or morphia, even cause the immediate death 

 of the organisms which they have enticed. However, as a rule 

 the substances which are hurtful to protoplasm generally repel 

 it ; this is the case with most acid and alkaline solutions. Even 

 2 per cent, solutions of citric acid and sodium carbonate exert a 

 distinctly repellent influence. 



Hence, within the above-mentioned limitations, the general rule 

 may be stated that organisms are, through positive chemotropism, 

 enabled to seek suitable nutriment, whilst in consequence of 

 negative chemotropism they avoid hurtful substances. 



These phenomena of chemotropism are of the greatest import- 

 ance in understanding many processes in the bodies of man and 

 of other vertebrates. Here also -there are cells which react to 

 chemical stimuli by changes of shape, and movements in special 

 directions. These cells are the white blood corpuscles and lymph 

 cells (leucocytes or wandering cells). 



The chemical irritability of leucocytes has been established as 

 a fact by the experiments of Leber (IV. 17a, 6) ; Massart and 

 Bordet (IV. 20, 21); Steinhaus (IV. 36); Gabritschevsky (IV. 

 10) ; and Buchner (IV. 2). If, in accordance with Pfeffcr's 



