94 BIOLOGY AND TECHNIQUE 



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

 Hill, 1 for the study of living bacteria in solid media is carried out as fol- 

 lows: 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 smearing, 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 



FIG. 14. HANGING DROP PREPARATION. 



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 paraf- 

 fin. The entire preparation can be placed upon the stage of a microsocpe. 

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



Living bacteria may also be studied in stained preparations by the 

 so-called "intravital" 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 transparent sky-blue color. The micro- 

 organisms 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 proc- 

 esses of heat or chemical fixation. According to Nakanishi, cytoplasm 

 xS stained blue, while nuclear material assumes a reddish or purplish 

 hue. 



The Study of Bacteria in Fixed Preparations. Stained preparations 

 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. 



(1) Smearing. Bacteria from a fluid medium are transferred in a 

 small drop of the fluid, 'with a platinum loop, to a cover-slip and care- 

 fully spread over the surface in a thin film. If taken from a solid medium 

 a small drop of sterile water is first placed upon the cover-slip and the 

 bacteria are then in very small quantity carefully emulsified in this drop 

 with the platinum needje or loop and spread in an extremely thin film. 



Hill, Jour, of Med. Research, vii, 1902. 



