94 BIOLOGY AND TECHNIQUE 



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

 Hill,^ 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 Dkop 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 grouni 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 

 is 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-sHps, 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.— B&ctena 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 needle or loop and spread in an extremely thin film. 



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



