128 NATURALISTS' ASSISTANT. 



Elder pith is used by a few as an embedding material for 

 some animal tissu?-, but I have had no experience with it. 

 The object, surrounded with pith, is placed in the tube of the 

 microtome and then the pith is wetted which causes it to swell 

 and hold the whole firmly. 



Of late years freezing has been a favorite method with those 

 objects which are injured by heat. In this process the speci- 

 men is immersed in a mucilage of gum arable in the tube of 

 the microtome and frozen by an ether spray, or by having the 

 tube surrounded by a tank in which is kept a freezing mixture 

 of salt and ice. 



Having embedded our object the next thing in order is to 

 cut the sections. The first thing necessary is a knife. The 

 most usual form is an ordinary razor, one side of which is 

 ground flat. It is well to have the other side concave as thus 

 a thinner edge is procured and the razor acts less like a wedge. 

 At other times a knife made especially for the purpose is used. 

 The knife, or razor, should be kept very sharp, and should 

 have a perfect edge, free from any nicks, even if too small to 

 be seen by the naked eye, as these will catch and ruin the 

 section. 



With practice good sections can be readily cut free hand, 

 and it is always desirable that the student should be able to 

 produce good results in this manner, whether he possess a 

 microtome or not. In cutting sections free hand the bit of 

 paraffine is held in the left hand and the razor either drawn 

 towards or pushed from the operator with the right. Some 

 work one way and some the other ; the razor should not be 

 pushed straight through but should have a drawing stroke, 



