Chapter II: General Statement of Methods 23 



drilling the student in the details of the two chief infiltration 

 methods, viz., the paraffin method and the celloidin methnd. 

 Infiltration with gum is also not infrequently resorted to, espe- 

 cially for tissues which would be injured by alcohol, or for 

 sectioning by the freezing method. 



Paraffin is a translucent, waxy material derived from various 

 sources, one of the commonest of which is crude petroleum. 

 Paraffins of low and of high melting-points, termed respectively 

 soft and hard paraffin, should be kept on hand so that mixtures 

 of different degrees of hardness may be made up as necessity 

 demands. 



Celloidin is a form of pyroxilin (gun cotton or collodion 

 cotton) specially prepared for interstitial imbedding. It is dis- 

 solved in a mixture of ether and alcohol (chap, i, reagent 4) and 

 solutions of two or three strengths are used for infiltration. For 

 details see the method, chap. vii. Collodion instead of celloidin 

 is used by some workers (see memorandum 11, chap. vii). 



AFFIXING SECTIONS 



When mounting sections upon a slide, especially if they are 

 yet to be stained, it is usually necessary to affix them firmly to 

 the slide to prevent later displacement. For paraffin sections 

 Mayer's albumen fixative (reagent 21, chap, i), or a combination 

 of this method with the water method, is most widely used. The 

 water method alone often proves adequate, particularly with thin 

 sections. The slide is flooded with water and the sections are 

 floated upon its surface. As the layer of water evaporates the 

 sections are slowly drawn down into close contact with the slide. 

 When perfectly dry they are usually so firmly affixed that they will 

 not become detached even after the removal of paraffin from them. 

 It is common, however, and safer to use a thin film of albumen 

 fixative as a cementing substance between the water and the 

 surface of the slide. 



In the case of celloidin sections, if only one or a few sections 

 are to be mounted on one slide, it is a common practice to stain 

 the sections and transfer them through the various reagents, even 



