392 GENERAL MICROSCOPIC 



tissues to be infiltrated are transferred to a mixture of equal 

 parts of absolute alcohol and ether. After remaining in this 

 for twenty-four hours they are transferred to the thinnest of 

 the three celloidin solutions, and left for forty-eight hours. 

 From this they are put into medium thick celloidin and left 

 for twenty-four hours ; then into thick celloidin for from 

 twenty-four to forty-eight hours. Large pieces of tissue should 

 be left in the celloidin solutions a longer time. After becoming 

 thoroughly saturated with celloidin the object is lifted out and 

 placed on a block of wood, or pressed wood fibre cut in a size 

 to fit the microtome. The object is put in the position desired 

 for sectioning, and a few drops of thick celloidin allowed to 

 harden over it. This fixes it firmly on the block, and sur- 

 rounds it with a layer of celloidin, which on evaporation 

 becomes firm. When the surface of the celloidin is somewhat 

 hardened, the block is placed in 70 per cent, alcohol until the 

 whole object is firm and hard. This usually requires from 

 twelve to forty-eight hours. With large pieces of tissue it is 

 better to place the object in a glass dish and cover it with 

 thick celloidin. This is allowed to evaporate slowly until the 

 whole dish of celloidin is firm and hard. The tissue is then 

 cut out together with a small quantity of the celloidin and 

 fastened to a block. 



For infiltration with paraffin, one requires pure paraffin, 

 which melts at. various temperatures. Two kinds are com- 

 monly used, a soft paraffin, melting at 45 C., and a hard 

 paraffin, melting at 56-57 C. There is needed also a ther- 

 mostat, or a paraffin oven, which is so arranged that the 

 temperature can be kept constant. 



The piece of tissue which is to be infiltrated with paraffin 

 should be entirely dehydrated in absolute alcohol. From this 

 it should be put into some fluid in which paraffin can be 

 dissolved. For this purpose xylol may be used. Tissues 

 are left in this for from ten to thirty minutes. Instead of 

 this, a mixture of 2 parts of absolute alcohol and 1 part of 

 chloroform may be employed for from two to twelve hours. 

 Pure chloroform may be used for from two to six hours. If 



