30 LABORATORY METHODS OF 



1. Take a 40 per cent solution of formaldehyd in 

 water or alcohol. Make a 10 per cent solution. The 

 celloidin solution is made by taking 5 grams of 

 celloidin and 100 grams, i. e., c.c., of the mixture of 

 equal parts of ether and absolute "alcohol. Mix. 



2. The thick celloidin is made by taking the same 

 mixture and adding celloidin until the mixture is as 

 thick as a sirup. 



By sectioning we mean cutting the tissue into very 

 thin sections, so that they can be easily made trans- 

 parent, by the next steps which are to follow. By 

 clearing a tissue we mean that we make the tissue so 

 transparent that it is of the same transparency as that 

 of the glass slide on which the tissue is mounted. Sev- 

 eral chemicals are used for this purpose. They all 

 depend on their indices of refraction for their value. 

 They are called clearing agents. 



A piece of tissue to be carefully and properly 

 mounted must go through the following steps : 



1. The cells must be killed. 



2. The cells must be fixed. 



3. The tissue must be hardened. 



4. The tissue must be dehydrated. 



5. The tissue must be dried. 



6. The tissue must be infiltrated. 



7. The tissue must be embedded, : 4 



