96 BACTERIOLOGY. 



For fixing directly on cork, small organs and pieces of firm tissue 

 such as the kidneys of a mouse, or liver, we may employ gelatine or 

 glycerine gelatine, liquefied over a Bunsen burner in a porcelain 

 capsule. Glycerine gelatine may be used with advantage for fixing 

 irregular pieces of tissue, as it does not become of a consistency 

 that would inj are the edge of the knife. The cork, with specimen 

 affixed, is placed in alcohol, and is ready for cutting sections next 

 clay. 



Material infiltrated with paraffine must be cut perfectly dry, 

 and the sections prevented from rolling up by gentle manipulation 

 with a camel's-hair brush. They must then be picked off the blade 

 of the knife with a clean needle, and dropped into a watch-glass 

 containing xylol. This dissolves out the paraffine. The sections are 

 then transferred to alcohol to get rid of the xylol, and then to the 

 staining solution. 



Staining Bacteria in Tissue Sections. Sections of fresh tissues 

 made with the freezing microtome are to be floated in '8 per cent, 

 salt solution, and then carefully transferred, well spread out on a 

 platinum lifter, to a watch-glass containing absolute alcohol. Simi- 

 larly, sections selected from those cut with Jung's microtome may 

 be transferred from the spirit to absolute alcohol. The sections 

 may be then stained by any of the methods to be described. 



It is often advisable to employ some method which will enable 

 one to study the structure of the tissue itself ; and sections, however 

 stained, should always be first examined with low powers, to enable 

 one to recognise the tissue under examination, and to examine in 

 many cases the topographical distribution of masses of bacteria. 

 With a power of about 250 diams. (one-sixth), very many bacteria 

 can be distinguished ; and with the oil immersion lenses the minutest 

 bacilli and micrococci can be recognised, and the exact form of 

 individual bacteria accurately determined. As most good modern 

 instruments are provided with a triple nose-piece, there is no loss 

 of time in examining a preparation successively with these different 

 powers. 



Weigert's Method. A very useful method for staining both 

 the tissue and the bacteria is as follows : Place the sections for 

 from six to eighteen hours in a 1 per cent, watery solution of any of 

 the basic aniline dyes (methyl violet, gentian violet, fuchsine, Bis- 

 marck brown). To hasten the process, place the capsule containing 

 the solution in the incubator, or heat it to 45 C. A stronger 

 solution may also be employed, in which case the sections are far 

 more rapidly stained, and are easily over-stained. In the latter case 



