THE STAINING OF COVER-GLASS PREPARATIONS. 97 



The coloration is generally even, that is to say, the 

 whole of the cell is pretty equally stained, there being 

 no distinction between the membrane and its contents. 

 A much less pretty result is obtained, if bacteria from 

 older cultivations are taken. For one thing, they take 

 up the stain much less readily, and hence are paler and 

 less distinct, and, besides, they do not become evenly 

 coloured ; one end of the rodlet may be deeply stained, 

 whilst the other is only faintly so, or colourless gaps 

 may occur in the cells, or round, deeply stained spots 

 may be seen. These colourless gaps are either spores 

 (i.e., the resting or permanent condition of the bac- 

 teria) which, as we shall see in a later chapter, are 

 very slightly affected by aniline dyes, or they may be 

 actual cavities in the cell. The plasma sometimes 

 shrinks up here and there, and cavities which do not 

 absorb the stain are formed between its intensely 

 stained particles. The membrane over these cavities 

 sinks in when the bacterium is dry, and as they 

 contain no substance capable of swelling, they do 

 not become extended again when the preparation is 

 soaked in watery staining solutions, so that it often 

 appears as though the bacterium were thinner at these 

 parts than at the coloured ones. 



There are also some bacteria (such as the bacillus 

 of fowl cholera) in which, even in the youngest cul- 

 tures, a bright streak is to be seen down the middle 

 of the cells in the stained preparations. This forms a 



H 



