534 BACTERIOLOGY. 



placed in cold water. The paraffin now sets thoroughly, 

 and after a few minutes the L's can be removed. The 

 block is now ready to be cut. 



When a number of paraffin blocks are prepared, and especially 

 if they are to be kept for some time, they should be labeled. This 

 can be done by placing the label, with the written side turned down, 

 in the bottom of the trough before pouring in the paraffin. The label 

 then adheres to the paraffin block. 



Imbedding in celloidin. — The tissue is transferred from 

 absolute alcohol to a mixture of equal parts of absolute 

 alcohol and ether for 24 hours. It is then placed in ether for 

 12 hours, and from this it is transferred to thin celloidin 

 where it remains for 2 to 3 days or longer. The tissue is 

 then placed in thick celloidin for an equal length of time 

 after which it is ready to be blocked. 



The end of a cork or similar block of wood is covered 

 with thick celloidin. This is allowed to partially evapor- 

 ate and the tissue is then placed on the block. After par* 

 tial drying in the air the block is placed in 80 per cent, al- 

 cohol for 24 hours. The material can then be sectioned or 

 it may be preserved in this alcohol for future use. 



Ordinary collodium may be used. The thick solution can be 

 readily obtained by allowing some of the collodium to remain in an 

 open wide-mouth bottle for some hours. The material can be thinned 

 whenever necessary by the addition of a mixture of equal parts of ab- 

 solute alcohol and ether. 



Cutting sections. — ^A good sliding microtome should be on 

 hand. This can be used for sectioning the material which 

 has been frozen, hardened in alcohol, or imbedded in paraf- 

 fin or celloidin. 



. The tissue which has been fixed and hardened in alco- 

 hol can be cut direct without resorting to the imbedding 

 process. The result, however, cannot be said to be as 

 good. To do this a piece of the tissue is attached to a small 

 cork by means of a glycerin gelatin mixture made by warm- 



