164 BACTERIOLOGY. 



with alcohol, so that the sections may float upon the 

 blade of the knife, from which they can be easily re- 

 moved, without tearing, with a curved needle or a 

 camel-hair pencil. As the sections are cut they are 

 placed in a dish containing alcohol. 



There are some tissues which, by reason of their 

 histological structure, do not become sufficiently dense 

 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/ 7 as this process is called. 



Imbedding in celloidin. Most convenient for this 

 purpose is celloidin, a body somewhat similar to collo- 

 dion, 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 in 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 solu- 

 tion, which should be of about the consistence 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 a thick solution for about 

 a day. From this it may be removed and placed imme- 

 diately 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 



