BACTERIOLOGICAL TECHNIQUE 31 



The above method is suitable for firm tissues and organs, but deli- 

 cate structures must be imbedded in celloidin, or frozen, before they 

 can be cut into sections. 



Formalin can also be used for hardening tissues, etc., but the pieces 

 must be very small, as formalin lacks the power of great penetration ; 

 however, small pieces of tissue, etc., can be laid in one part of 40 per 

 cent, formalin and ten parts of water for six to eight hours, and then 

 placed in absolute alcohol. 



XXXII. METHOD OF EMBEDDING IN CELLOIDIN. 



1. Transfer the hardened tissue or organ from the alcohol into 

 equal parts of alcohol and sulphuric ether for twelve hours. 



2. Transfer the tissue, etc., into a solution of celloidin made with 

 equal parts of alcohol and sulphuric ether and enough celloidin to 

 form a thin syrup ; small pieces of tissue, etc., remain in this solution 

 two days, while larger pieces remain four or five days. 



3. Remove the piece of tissue and place in a small porcelain 

 evaporating dish, pour in some of the lower or thicker solution of 

 celloidin until the tissue is covered, when the dish is set aside until 

 the celloidin is quite hard ; at least twenty-four hours, sometimes 

 longer, are required. 



4. Cut the celloidin away from the sides of the porcelain dish, 

 place the imbedded tissue on a block, and fasten with a little celloidin 

 solution. Expose the block to the air until the celloidin hardens 

 about two hours and then place in 80 per cent, alcohol, and keep 

 until required. Absolute alcohol must on no account be used, as 

 it dissolves the celloidin. 



XXXIII. METHOD OF PREPARING TISSUES FOR 

 FREEZING. 



The method of cutting sections with the freezing microtome is con- 

 ducted with a special apparatus sold with most of the modern micro- 

 tomes, consisting of an ether spray and special platform for holding 

 the block of tissue. A small microtome, manufactured by Jung of 

 Heidelberg, known as the Students' Microtome, is specially adapted for 

 frozen objects and those imbedded in paraffin, being fitted with a 

 mechanical knife-guide, enabling the most inexperienced to make 

 serial sections without difficulty (see Fig. 8). 



