INFILTRATION GF TISSUE WITH CELLOIDIN AND PARAFFIN. 393 



xylol is used, the tissues are transferred to a saturated solu- 

 tion of paraffin in xylol for two hours, and from this to melted 

 soft paraffin. If a chloroform solution is used, the tissues are 

 placed for twelve hours in a saturated solution of paraffin in 

 chloroform, and from this to soft paraffin. In the latter sub- 

 stance the tissues are left for from one-quarter to two houiv, 

 and then transferred to melted hard paraffin, and left in this 

 for from one-quarter to two hours. They should be allowed 

 to remain in the hot paraffin as short a time as is possible for 

 their complete infiltration. For small pieces, fifteen minutes 

 are sufficient. In cold weather hard paraffin is difficult to cut, 

 although thinner sections can be made with hard than with 

 soft paraffin. When the tissues have become thoroughly 

 infiltrated with the paraffin, they are placed together with a 

 quantity of paraffin in a small paper box, which may be made 

 by folding ordinary paper to the required shape; or they may 

 be placed in a watch glass the bottom of which has been covered 

 with a little glycerin. The pieces of tissue are placed in their 

 proper position, and the whole is cooled quickly in cold 

 water. 



The method of cutting sections with the microtome differs 

 somewhat according to the way in which the object has been 

 imbedded. Tissue imbedded in celloidin is fastened, as de- 

 scribed, to a block of wood or wood fibre, and fixed firmly in 

 the microtome. The knife is placed in the knife-holder of the 

 microtome, so that it is drawn through the tissue very obliquely. 

 It should be slanted so much that nearly the whole edge of the 

 knife passes through the piece of tissue. The surface of the 

 knife should be kept flooded with 70 per cent, alcohol, and the 

 object also should be kept moist. The sections which are cut 

 are floated out in the alcohol on the knife, and are removed 

 by means of a camel's hair brush to a vessel of 70 per cent, 

 alcohol, where they may be kept for a considerable time 

 without being injured. 



Tissues imbedded in paraffin should be fastened to a block 

 by melting the lower surface of the mass of paraffin. The knife 

 should be placed with the edge at right angles to the long axis 



