PREPARATION OF SPECIMENS 7 



are about equal in thickness. The filled mold is now 

 rapidly cooled by immersion in cold water, care being 

 taken to see that the water does not flow over the sur- 

 face of the paraffin until a fairly thick pellicle is formed. 

 Rapid cooling is necessary in order to prevent crystal- 

 lization of the paraffin, as this would cause chipping 

 and crumbling during cutting. After the paraffin is 

 throughly hardened the mold is removed, and the 

 block containing the specimen is trimmed so that 

 approximately equal thicknesses of paraffin surround 

 the specimen on all sides. The blocks are most satis- 

 factory when they are small; their length should not 

 exceed 25 mm. 



The placing of the specimen in the block depends 

 upon the, view or aspect of the object one desires. 

 Transverse or cross-sections are cut parallel to a plane 

 extending at right angles to the long axis of the speci- 

 men. Radial or radial-longitudinal sections are cut 

 parallel to a plane extending through the long axis of 

 the specimen and passing through the center of the 

 object. Tangential sections are cut parallel to a plane 

 extending through the long axis of the specimen but 

 not passing through the center. 



SECTION CUTTING 



Free-hand Sections. — The simplest method of 

 obtaining sections is by means of the section razor or 

 hand microtome. This instrument is similar to a 

 heavy razor except that it is flat on one side and con- 

 cave on the other. Although with practice one may 

 readily cut a small number of fairly good sections by 

 this means, it is difficult to secure uniformity as regard^ 

 thickness and size of sections. The block containing 



