25 



and longer than high. Watch glasses, watch crystals, small tin pans, 

 etc., may be used as imbedding receptacles. 



57. Crystallization of the imbedding mass. Paraffin that has crystallized 

 is crumbly and will not give good sections. When crystallization has occurred, 

 it is best to reimbed. Its occurrence is usually due to too slow a cooling of the 

 imbedding mass, or cooling under conditions that prevent the paraffin from shrink- 

 ing. It is sometimes due to the presence of impurities in the paraffin, such as 

 water (?), excess of clearer, etc. 



58. Cutting the sections. The essentials for good paraffin 

 sectioning are (1) well-imbedded tissue, (2) a sharp microtome knife 

 (or section razor), ( 173), (3) a room of the proper temperature, and 

 (4) the paraffin block properly trimmed and arranged in the micro- 

 tome. Furthermore, tissues fixed and hardened in different ways cut 

 very differently. Tissue fixed in Hermann's, Flemming's, Mailer's, 

 Zenker's fluid or Carnoy's fluid, etc., cuts well; alcohol and mercuric 

 chlorid tissue is more apt to be tough or hard, etc. The different 

 organs and tissues have of course very different adaptabilities to the 

 method. 



After the imbedding mass is well cooled, remove the paper box 

 and trim the part containing the tissue in a pyramidal form, two of 

 the sides at least being as nearly parallel as possible. Clamp the 

 block of paraffin in the holder of the microtome so that the tissue 

 will be at the proper level for cutting, being careful to have the par- 

 allel sides also parallel to the edge of the knife. If a ribbon micro- 

 tome is used, heat the holder and melt the end of the block upon it. 

 Cool and place the holder in its place in the microtome, again hav- 

 ing the parallel sides and the edge of the knife parallel. Use a very 

 sharp, dry section knife for cutting the sections. Clamp it in the 

 microtome slightly inclined to the cutting surface of the tissue. If 

 the temperature of the room is right for the paraffin used, the sec- 

 tions will remain flat, and if the directions given above for trimming 

 and arranging the block be observed, they will adhere and thus form 

 a ribbon. If the room is too cold or the paraffin too hard, the sec- 

 tions will roll; if it is too warm, the sections will crush or be imper- 

 fect. If a microtome in which the knife is not fixed, is used, make 

 the sections with a rapid straight cut as in planing. Do not try to 

 section with a drawing cut as used in celloidin sectioning. 10^ will 

 be found the most convenient thickness for the sections, though in 

 special cases they should be thinner or even thicker. Handle the 

 sections by means of a camel's hair brush, a needle, or sometimes on a 

 scalpel handle, when cutting ribbons, etc. 



