STAINING OF THE SECTIONS. 165 



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 

 this they are transferred to pure turpentine, and kept in 

 a warm oven at a temperature not exceeding 35° to 

 38° C. Here they remain for a time sufficient for them 

 to become thoroughly saturated with the turpentine, as 

 is recognized by the transparent appearance that they 

 assume. From this they are placed in paraffin that is 

 melted at 53° C, and allowed to remain in this for 

 three or four hours. They are then transferred to a 

 small paper or metal mould, or a pill-box, and melted 

 paraffin is poured over them. When the paraffin has 

 become solid the mould or pill-box is removed from 

 around it, the excess of paraffin removed from about 

 the imbedded tissue, and the latter is ready for cutting. 



When the sections are cut they are freed from par- 

 affin by exposing them to turpentine; the latter is re- 

 moved by washing in alcohol and the sections can now 

 be stained in the ordinary way. In cutting sections 

 from tissues that have been imbedded in paraffin the 

 long axis of the knife should be at nearly right angles 

 to the direction in which the knife travels. For bacte- 

 riological purposes the method of imbedding in paraffin 

 does not, as a rule, give such good results as when the 

 celloidin method is employed. In this work, therefore, 

 the latter is usually preferred. 



Staining of the Sections. — The sections when cut 

 may be stained in a variety of ways. The ordinary 



8* 



