API'ENDIX 223 



Collodion. — Mixed with an equal part of oil of cloves. 

 Used for fixative in the same way as shell-lac and alcohol. 



IV. METHODS OF PREPARING MATERIAL FOR MICRO- 

 SCOPIC EXAMINATION. 



In fully equipped laboratories the teacher and student will 

 have at command special books on microscopical technique 

 and methods, but for the benefit of those who may not 

 have access to such books the following brief suggestions 

 are given : 



The particular methods to be used depend upon the nature 

 of the tissue and the kind of observation to be done. As a 

 rule, the softer tissues have to be hardened, and the harder 

 softened or cleaned, before making permanent mounts. 



1. Killing: The method of killing depends upon the 

 animal, but as a rule that method should be used that will 

 leave the body in natftral form with tissues uninjured. 

 Chloroform for the crustaceans and vertebrates, potassium 

 cyanide for the insects, spiders, and centipedes, warm water 

 for the mollusks and earthworms, can in general be rec- 

 ommended; and for lower forms special methods may be 

 found by consulting books on histology. 



2. Hardening: For ordinary purposes alcohol is the 

 best hardening agent. The specimen should be placed first 

 in weak solution, about sixty per cent., then in seventy per 

 cent, then eighty per cent, and ninety per cent, and finally, 

 before beginning the work of imbedding, it should be placed 

 in absolute or one hundred per cent, alcohol. Considerable 

 alcohol should be used each time, except the last, and the 



