INFILTRATION AND IMBEDDING. 3! 



The hardened tissues are cut into small pieces, which should not 

 be much more than y% of an inch in thickness and not have a surface 

 area of more than ^ of a square inch. Much larger pieces of tissue 

 may be imbedded in celloidin. This is not advised, however, unless it is 

 necessary to show the whole of the structure to be studied. The pieces 

 to be imbedded are placed for twenty-four hours in absolute alcohol, and are 

 then transferred for twenty-four hours to a mixture of equal parts of abso- 

 ute alcohol and ether. Then they go into the thin celloidin solution, where 

 they remain for from twenty-four hours to several days, depending on the 

 size and density of the pieces to be imbedded. The pieces of tissue are 

 then transferred to the thick celloidin solution, where they again remain 

 for from twenty-four hours to several days. If it is desired to imbed large 

 pieces, especially if these be of the medulla or brain, the stay in the cel- 

 loidin solutions should be lengthened to several weeks. The hardening 

 of the celloidin may now be obtained by one of several methods. 



A sufficient quantity of the stock or thick celloidin solution to 

 cover well the tissues to be imbedded is poured into a flat dish large 

 enough to allow the pieces to be imbedded to be arranged on its bottom 

 and leave a space of about ^ of an inch between adjacent pieces. The 

 dish is then covered, not too tightly, and set aside to allow the ether and 

 alcohol to evaporate. In one or two days the celloidin is usually hard 

 enough to cut into small blocks, each block containing a piece of the 

 imbedded tissue. The blocks of celloidin are now further hardened by 

 placing them in 80% alcohol. A stay of several hours in this alcohol is 

 usually sufficient to give them the hardness required for section cutting. 

 After the celloidin pieces have obtained the right degree of hardness they 

 are to be stuck to small pieces of pine wood or vulcanized fiber so that they 

 may be clamped into the microtome. This is done in the following way : 

 A piece of celloidin containing a piece of tissue is trimmed with a sharp 

 knife so that only a rim of celloidin about ^ of an inch in thickness 

 surrounds the piece of tissue. It is now placed for a few moments in the 

 ether and alcohol solution. This is to soften the surfaces of the celloidin. 

 One end of a small pine-wood or vulcanized-fiber block about one inch long, 

 the cut end of which has a surface area slightly larger than the celloidin 

 block, is dipped for a few moments into the ether and alcohol solution and 

 then into the thick celloidin. The celloidin block is now taken from the 

 ether and alcohol solution, dipped into the celloidin, and pressed against 

 the end of the wooden or vulcanized-fiber block, which has been coated 

 with the celloidin. The whole is now set aside for a little while to allow 

 the celloidin to harden slightly, and is then placed in 80% alcohol. In 

 the alcohol it may remain indefinitely ; it may, however, be used for 

 cutting as soon as it again becomes hard. 



The piece of tissue to be imbedded may be mounted at once on 

 pine-wood or vulcanized-fiber blocks from the thick celloidin solution by 

 pouring a small amount of thick celloidin over one end of the block and 

 placing the piece of tissue from the thick celloidin solution onto the layer 

 of celloidin on the block. In three to four minutes a layer of the thick 

 celloidin solution is poured over the piece of tissue and the end of the 

 block. It may be necessary to do this several times if the piece of tissue 

 is large or of irregular shape. The block is now set aside for about five 

 minutes, and is then placed in 80% alcohol, where it remains until the 

 celloidin is hard, or until it is desired to cut sections. 



