METHODS OF CULTURE AND OF EXAMINATION. 103 



continue in active movement, and in many of them, as a 

 result of the action of the stain, even the flagella become 

 visible. In making examinations in hanging drop it is 

 advisable first to find the border of the drop with a low 

 power of the microscope, and then to scrutinize it with 

 higher powers. After having observed sufficiently the size 

 and the shape of the bacteria in a thin layer, and while 

 comparatively quiet, the central portions of the drop are 

 brought into the field. 



Instead of a simple hanging drop, a culture may be made 

 in hanging drop, and the growth of the individual bac- 

 terial cell, the formation of spores and their germination, 

 etc., can be observed directly under the microscope. As 

 a matter of course the cover-slip must previously have 

 been sterilized by being passed through the flame. After 

 the slip has cooled, a drop of sterile bouillon or gelatin is 

 placed at its center and is inoculated. The observation 

 may then be continued, perhaps with the aid of a warm 

 stage, or of a special small incubator in which the entire 

 microscope is introduced. 



In order to decide with certainty whether the movement 

 observed in certain bacteria is dependent upon their own 

 motility, upon molecular movement, or upon that due to 

 currents, it is at times necessary to employ liquid gelatin as 

 a diluting fluid instead of sterile water. The degree of 

 temperature necessary for liquefaction is readily obtained 

 by using the warm stage. All such observations are, of 

 course, made through a narrowed diaphragm. 



Examination of Stained Preparations. For staining 

 purposes basic aniline dyes are employed that possess the 

 property of staining nuclei and bacteria. The most impor- 

 tant of those in use are gentian -violet, methyl-violet, fuch- 

 sin, methylene-blue, vesuvin, and malachite-green. These 

 stains, with the exception of the last two, are kept in readi- 

 ness in concentrated alcoholic solution, and the two ex- 

 cepted, vesuvin and malachite-green, in about one per cent, 

 watery solution. From the alcoholic stock-solutions the 

 usual watery staining solutions are prepared by dilution 

 with from ten to twenty times the amount of distilled water. 

 A still simpler means consists in permitting a few drops of 

 the stock -solution to pass through a filter into a watch-glass 

 containing distilled water. By the addition of certain sub- 

 stances that, to a certain extent, play the part of mordants, 



