Chapter V: The Paraffin Method 43 



NOTE. As time permits, cut the other sections which are 

 imbedded in paraffin. When, as in the present case, it is not 

 necessary to have a complete series of sections, you may place 

 fewer sections on a slide and use smaller covers. 



When a small cover is to be used, place the sections at the 

 center of the slide. The center may readily be determined by 

 drawing the outline of a slide on a card and connecting the oppo- 

 site corners of the figure by means of diagonal lines. When 

 mounting, place a slide over the diagram ; the intersection of the 

 diagonals shows the center. 



At this point the student should make a careful study of 

 Appendix A if he is not already thoroughly acquainted with the 

 optical principles involved in microscopy. 



MEMORANDA 



1. If Paraffin Becomes Dirty it should be meited and filtered. 



2. Oil of Cedar, if used for dealcoholization before imbedding, should 

 be followed by at least two changes of paraffin or the paraffin does not 

 thoroughly replace the oil and the object is likely to drop out of the sec- 

 tions as they are cut. In my experience this is the commonest difficulty 

 which beginners encounter if they use cedar oil for dealcoholization. 

 For this reason xylol is recommended as preferable for general work. 



3. Objects Imbedded in Paraffin may be preserved in that form indefi- 

 nitely. It is one of the most convenient ways, in fact, of preserving mate- 

 rial which is to be sectioned in paraffin. 



4. Small White Objects, if not stained before imbedding, should be 

 tinged with a dilute solution of Bordeaux red to facilitate orientation. 

 For orientation in general see chap, xvi, memorandum 12. 



5. With Delicate Tissues it is necessary that the transition from alco- 

 hol to clearer be gradual, hence it is best to add the clearer, a little at 

 a time, to the last alcohol, transferring it with a pipette to the bottom of 

 the alcohol. 



6. The Temperature of the Laboratory must be taken into account 

 when sectioning in paraffin. In summer use a harder, in winter a softer^ 

 paraffin. 



7. For Thin Sections use a hard paraffin, for thick sections, a softer 

 paraffin. 



8. For Valuable Tissues Which Crumble in Paraffin Alone the following 

 somewhat tedious process (Mark, American Naturalist [1885], p. 628) 

 may be resorted to. Prepare a very fluid collodion in ether-alcohol and 



