MICROSCOPICAL PREPARATIONS. 5 



and spread out to form a thin layer, by placing a cover glass 

 on top of it, or else the drop may be placed inside a moist 

 chamber (described later) in which the growth and propaga- 

 tion of the cells can be followed. For instance, by such 

 observations of the living cell, the development of spores in 

 the yeast cell may be observed, and the difference in shape 

 of the ripe spore in individual species, the thickness of the 

 wall of the mother-cell, etc., may be noted. 



Certain characters, however, of the detailed construction 

 of these organisms can only be detected by the use of special 

 drying and staining methods. To do this, the cells are sub- 

 jected to a thorough treatment with concentrated dyes, some 

 of a poisonous character. They are killed, and the possibility 

 exists that the characters brought out by staining may differ 

 somewhat from those of the living cell. Drying may also 

 modify the length and breadth of the bacteria. On the other 

 hand, the staining process has explained many phenomena 

 which were not apparent by observation of the living cell. 



Dilute dyes (e.g., eosin, methylene blue) are used in the 

 technical examination of yeasts to obtain an idea of the 

 proportion between dead and living cells in a vegetation, 

 since the dead cells alone absorb the dye. 



As an example of a method of staining, we may instance the 

 treatment to which yeast cells are subjected in order to observe 

 the cell-nucleus and its subdivision. Hoffmeister proceeds as 

 follows : The young, vigorous yeast growth is washed several 

 times with distilled water, and then before the actual staining 

 it is subjected to the process of fixing, according to one of the 

 recognised methods. For instance, the yeast is stirred up 

 with Rath's solution (consisting of a litre of a concentrated, 

 aqueous solution of picric acid, together with 4 c.c. of glacial 

 acetic acid and 1 gramme of osmic acid) ; the cells are thus 

 coloured yellow. After allowing the mordant to react for 

 24 hours, the cells are again washed with water, spread out 

 in a thin layer on a cover-glass, and allowed to dry. The 

 preparation is then treated according to Heidenhain's method. 

 The cover-glass preparation is allowed to float in a Petri dish, 

 on the surface of a solution containing 2-5 per cent, of iron 

 alum ; after 6 to 24 hours the cover-glass is washed once with 



