STAINING OF BACTERIA IN TISSUES. ]47 



when exposed to alcohol to permit of their being cut in 

 the above way. It becomes necessary to render them 

 more solid by filling their interstices with some sub- 

 stance that neither interferes with their structure nor 

 prevents their being cut into sections. They must be 

 " imbedded," as this process is called. 



Imbedding in cdloidin. Most convenient for this 

 purpose is celloidin, a body somewhat similar to col- 

 lodion, soluble in a mixture of equal parts of alcohol 

 and ether, as well as in absolute alcohol. 



After hardening in alcohol the tissue to be imbedded 

 is placed into a mixture of equal parts of absolute 

 alcohol and ether and left there for twenty-four hours. 

 It is then transferred to celloidin. Two solutions of 

 celloidin are to be employed, the one a thin solution in 

 a mixture of equal parts of absolute alcohol and ether, 

 the other a thick solution in the same solvent. Into 

 the thin solution, which should be of about the consist- 

 ence of very thin syrup, the tissue is placed from the 

 absolute alcohol and ether, and allowed to remain there 

 for twenty-four hours. It is then placed in the thick 

 solution for about a day. From this it may be removed 

 and placed immediately upon a bit of cork or a block 

 of wood. The adherent celloidin will act as a cement, 

 and as it hardens rapidly, the tissue is soon fast to the 

 cork. It is then left in 60 per cent, alcohol for twenty- 

 four hours to complete the solidification of the celloidin 

 after which sections may be cut in the way just described 

 for tissues not so treated. 



Imbedding in paraffin. After bits of the tissue not 

 larger than a cubic centimetre have been hardened in 

 the usual way, they are placed in fresh absolute alcohol 

 for twenty-four hours to complete the process. From 



