16 THE AMERICAN MONTHLY [January, 



of motile bacteria has been made possible, and, in many instances, com- 

 paratively simple. The discovery of a process by which these minute 

 structures could be stained was first made by Koch, who, after a 

 long series of negative experiments made with the ordinary dyes em- 

 ployed in staining bacteria, finally succeeded in staining the flagella on 

 a certain number of saprophytic forms by using a solution of the extract 

 of logwood. Since then other methods have been devised for staining 

 the flagella on pathogenic as well as saprophytic bacteria, so that now we 

 are able to demonstrate their presence on nearly all known motile germs. 

 The development of these methods has been attended with much interest, 

 for each step in advance has been taken only after long and patient study, 

 and each in its turn has been to a greater or less degree the foundation 

 for the succeeding one. In reviewing the various methods, I shall give 

 them in a form as condensed as possible, assuming that the ordinary 

 technique in staining bacteria is somewhat familiar. It is but just, how- 

 ever, to note that each author to be referred to gives a very full account 

 of his experiments, and, in nearly every case, the experiments and re- 

 sults which led to the formulation of the method finally recommended. 

 Although of much interest in the history of the development of the 

 latest methods, the unsatisfactory processes will, as a rule, be omitted at 

 this time. 



Koch^s Method. — Koch (4) employed as his staining fluid a concen- 

 trated aqueous solution of the extract of logwood. 



The bacteria and their flagella were stained by applying this staining fluid in 

 two diflTerent ways : (i) By adding the solution to a small quantity of the germ- 

 containing liquid. (2) By making cover-glass preparations, by the oi'dinary 

 method, from the germ-containing substance, allowing them to dry in the air, 

 after which the film on the cover-glass was covered with the staining solution 

 and allowed to remain for a considerable length of time. The cover was then 

 rinsed in water and the preparation mounted in glycerine. The flagella were 

 stained a beautiful brown. In order to retain the flagella in a stained condition 

 the preparations were treated, prior to mounting, with a weak solution of 

 chromic acid or with Miiller's fluid. The stained cover-glass could be allowed to 

 dry and mounted in balsam. By this process Koch preserved a specimen of the 

 bacillus tretnulus which showed both the spores and flagella. 



Neuhauss' Method (5). — This method was devised with special ref- 

 erence to the staining of the flagella of the comma hacilhis. It is as 

 follows : 



The dried cover-glass preparations of a culture of this germ were boiled five 

 minutes in common (Kaiser) black ink, after which they were placed in a weak, 

 warmed solution of chromate of sodium for fifteen minutes. This process was 

 repeated two or three times. Upon examination very delicate lines could be 

 seen extending from the ends of some of the bacilli, but they were so indis- 

 tinct that the author could not positively afiirm that they were flagella. It was 

 this failure to satisfactorily stain the flagella of these germs that led Neuhauss to 

 employ photography as a means of demonstrating their presence, as previously 

 noticed. 



Loeffler's Method. I. — Prof. Loeffler (6) was the fir.st to apply the 

 principle of a mordant in the staining of the flagella and cilia of micro- 

 organisms. He succeeded in staining the flagella on a large number 

 of motile bacteria by subjecting the prepai"ations to the action of a 

 mordant before they were brought into the staining fluid. The pro- 

 cedure recommended is essentially as follows : 



