BIOLOGICAL METHODS 21 



life of a cell. Bacteriologists are not content to study the 

 shape and the reactions of microbes in connexion with 

 certain dyes, but also examine the aspect of their colonies, 

 their action on the culture-medium, the poisons they secrete, 

 their susceptibility to different antiseptics, the nutritive 

 substances which they require, etc. The same is true of 

 tissue-cells. Through our new techniques a cell type can 

 be characterized by the appearance of its colonies, its mode 

 of locomotion as recorded by cinematography, its effect on 

 the coagulum, its rate of growth, the nature of the sub- 

 stances which inhibit cell-multiplication, the nature and 

 concentration of the nutrient substances, etc. 



'The new cytology permits the identification of cells and 

 prediction of their conduct under given conditions. It 

 reveals the specific properties of each type of cell. Thanks 

 to it the mechanism of complex phenomena which take 

 place in the normal or pathological tissues can be sub- 

 mitted to experimental analysis. Its fecundity will neces- 

 sarily be greater than that of the classical cytology.' 



The explanted tissues, kept alive, can be assimilated to 

 ideal experimental animals. First, because they are simple and 

 deprived of a nervous system; secondly, because they can be 

 manufactured at will in large quantities. It is thus possible 

 to experiment indefinitely on the same family of cells proceed- 

 ing from the same stock. This eUminates innumerable causes 

 of error due to the individual characteristics of animals of 

 different origin. Furthermore, at each stage of an experiment, 

 one disposes of a permanent control, since it is derived from 

 the original culture itself, cut in two equal parts. 



We have outlined a cursory view of certain methods which 

 can be classed in the second category. We see that the unit 

 is no longer the individual, but the cell, and that the principal 

 tool of investigat on is the microscope, as in the case of 

 bacteriology. The limit of observation is what is called the 

 resolving power of the microscope, that is to say the order of 

 magnitude of the smallest objects which can be clearly 



