IMBEDDING IN CELLOIDINE 97 



solve out the paraffin, after which they are placed 

 in absolute alcohol, and thence into water. If they do 

 not sink in water, the paraffin has not been properly re- 

 moved ; in this case they are put back to soak in alcohol 

 and xylol. 



Imbedding in Celloidine. — The alcohol-hardened portions 

 of tissue are fixed to bits of cork by means of a solution of 

 celloidine in a mixture of alcohol and ether ; and then, 

 after the celloidine has set, they are immersed in absolute 

 alteohol for some time (about twenty-four hours), when they 

 become of a suitable consistence for cutting. The pieces 

 are now soaked in a mixture of alcohol and ether, and 

 finally in a celloidine solution of medium consistency, in 

 which they remain for about twenty-four hours. The pre- 

 pared pieces are now allowed to dry in the air for a short 

 time, and then immersed in 30 per cent, alcohol for three 

 or four days. The celloidine becomes first cloudy and then 

 converted into an opaque, milk-white mass, which is of a 

 sufficient consistency to be cut by the microtome. By 

 this method the sections are saturated, so to speak, with 

 celloidine, which is capable of taking up the stain in the 

 same manner as the actual tissue. 



Very fine sections are obtained by this method even with 

 the most refractory materials ; for instance, satisfactory 

 preparations of actinomyces in sections can be obtained 

 by this process, which cannot be obtained by any other 

 method. 



The Staining of Bacteria in Sections. — There are a great 

 number of different methods published for the staining of 

 bacteria in sections of tissue. The following procedure is 

 common to most of the published methods : The sections 

 are transferred from the alcohol to water ; they are then 

 subjected to the action of the stain, which varies from a 

 few minutes to several hours. The time is in some cases 



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