12 HISTOLOGIC.U. TECHNIC 



Solution No. i. Thin celloidin — made by diluting solution No. 2 

 with an equal volume of equal parts of alcohol and ether. 



The hardened tissues are placed successively in : 



Strong alcohol (96-per-cent.) twelve to twenty-four hours, to 

 dehydrate. 



Equal parts alcohol and ether, twelve to twenty-four hours. 



Thin celloidin, twenty-four hours to several days. 



Thick celloidin, twenty-four hours or longer. 



The exact time tissues should remain in the different grades of 

 celloidin depends upon the character of the tissue, the size of the 

 specimen, and the thinness of section desired. Many tissues may be 

 advantageously left for weeks in thin celloidin. 



The specimen must now be "blocked" and the celloidin hardened. 

 By blocking is meant fastening the specimen impregnated with cel- 

 loidin to a block of wood or other suitable material which may be 

 clamped in the microtome (see Section Cutting, p. 14). The specimen 

 may be taken from the thick celloidin (considerable of the latter 

 adhering to the specimen) , quickly pressed upon a block of wood or 

 vulcanized fibre, allowed to harden five to ten minutes in air, and then 

 immersed in 80-per-cent. alcohol. The alcohol gives an even hard- 

 ening of the celloidin, attaching the specimen firmly to the block. 

 Another method, and one by which very even-shaped blocks may be 

 obtained, is to place the specimen from the thick celloidin into a little 

 paper box (made by folding paper over a wooden block), sHghtly 

 larger than the specimen, and covering with thick celloidin. The 

 celloidin should dry slowl}' under a bell-jar for from two to twelve 

 hours, according to the amount of celloidin, after which it should be 

 immersed in 80-per-cent. alcohol and the paper pulled oft". Such a 

 block may be cut into any desired shape. It is attached to the 

 w^ooden or vulcanized block by dipping for a moment in thick celloidin, 

 and then pressing firmly down upon the block. After five to ten 

 minutes' drying in air it is transferred to 80-per-cent. alcohol. 



Old, hard, celloidin-embedded specimens are sometimes difficult 

 to attach to blocks. This usually may be accomplished by first 

 thoroughly drying the specimen and then dipping it in equal parts 

 alcohol and ether for a few minutes. This softens the surface of the 

 celloidin, after which the specimen is dipped in thick celloidin and 

 blocked. Embedded or blocked specimens can be kept in 80-per- 

 cent. alcohol. After several months, however, the celloidin is likely 

 to become too soft for good section cutting. In that case the speci- 



