112 BIOLOGY AND TECHNIQUE 



the slide in such a way that the drop hangs freely within the hollow 

 space. The preparation is then ready for examination under the 

 microscope. 



Another method, known as the "hanging block method," devised 

 by Hill, 2 for the study of living bacteria in solid media is carried 

 out as follows : nutrient agar is poured into a Petri dish and allowed 

 to solidify. Out of this layer a piece about a quarter of an inch 

 square is cut. This is placed on a sterile slide. The upper surface 

 of the agar block is then inoculated with bacteria by surface smear- 

 ing, and the preparation covered with a sterile dish and allowed 

 to dry for a few minutes in the incubator. A sterile cover-slip is 

 then dropped upon the surface of the block and sealed about the 

 edges with agar. Block and cover-slip are then taken from the 

 slide and fastened over a moist chamber with paraffin. The entire 

 preparation can be placed upon the stage of a microscope. This 

 method is especially designed for the study of cell-division. 



FIG. 9. HANGING-DROP PREPARATION. 



Living bacteria may also be studied in stained preparations by 

 the so-called " intra vital' ' method of Nakanishi. Thoroughly 

 cleaned slides are covered with a saturated aqueous solution of 

 methylene-blue. This is spread over the slide in an even film and 

 allowed to dry. After drying the slide should appear of a trans- 

 parent sky-blue color. The microorganisms which are to be examined 

 are then emulsified in water, or are taken from a fluid medium and 

 placed upon a cover-slip. This is dropped, face downward, upon 

 the blue ground of the slide. In this way bacteria may be stained 

 without being subjected to the often destructive processes of heat 

 or chemical fixation. According to Nakanishi, cytoplasm is stained 

 blue, while nuclear material assumes a reddish or purplish hue. 



The Study of Bacteria in Fixed Preparations, Stained prepara- 

 tions of bacteria are best prepared upon cover-slips, the process 

 consisting of the following steps: (1) Spreading on cover-slip; (2) 

 drying in air; (3) fixing; (4) staining; (5) washing in water; (6) 

 blotting; (7) mounting. 



2 Hill, Jour, of Med. Eeseareh, vii, 1902. 



