LABORATORY DIRECTIONS. 461 



Tissues are left in melted paraffin from two to six 

 hours. 



(c) Solidification of paraffin may be accomplished 

 by means of 



(1) Watch glasses. 



(2) Metallic frames. 



(3) Paper trays. 



(4) Tea-lead trays. 



The paraffin is poured into these trays, and the 

 tissue quickly transferred to it. The piece is then 

 oriented, and as soon as the paraffin has cooled 

 enough to form a crust, the whole block is placed in 

 cold water, where the paraffin is quickly cooled so 

 as to avoid crystallization. The block is then ready 

 to be cut in sections. 



A. Advantages of Celloidin Imbedding. 



1. No heat is necessary. 



2. Large sections may be cut, because 



penetration is more complete than 

 is the case with paraffin. 



3. Unnecessary to remove celloidin to 



stain; sections are therefore easily 

 handled. 



B. Disadvantages of Celloidin Sections. 



1 . Slow process at least three days. 



2. No thin sections. 



3. Celloidin may take the stain, particu- 



larly with saffranin. 



4. Serial sections difficult to make. 



5. More expensive. 



