EXAMINATION OF BACTERIA WITH THE MICROSCOPE. 49 



Imbedding in Paraffin. (a) Pieces of tissue 2 to 3 mm. 

 thick which have already been fixed in alcohol or formal- 

 dehyde are to be placed in absolute alcohol for twenty-four 

 hours. 



(b) In pure xylol one to three hours. 



(c) In a saturated solution of paraffin in xylol one to 

 three hours. 



(d) In melted paraffin having a melting-point of 50 C, 

 which requires the use of a water-bath or oven, one to three 

 hours. The xylol must be entirely driven off, and the tis- 

 sue thoroughly infiltrated. 



(e) Change to fresh paraffin for one hour. 



(/) Finally, place the tissue in a small dish or paper 

 box and pour the melted paraffin about it. Harden as 

 quickly as possible with running water. It is important to 

 fix the piece of tissue in a suitable position, if the position 

 is of importance, before pouring in the melted paraffin. 



Sections of exquisite thinness may now be cut. The 

 knife need not be wet. Paraffin imbedding is especially 

 desirable when serial sections are to be made. 



In order to mount the sections, proceed as follows : 



(a) Place the sections on water in a porcelain capsule. 

 Warm slightly, when the sections will flatten nicely. Smear 

 the surface of a slide with a very thin layer of Mayer's 

 glycerin-albumen mixture. Dip the slide under the sec- 

 tions; lift them; and then drain off the water, leaving the 

 sections in their proper positions. Let them dry for some 

 hours in the incubator, and they will be firmly fastened to 

 the slide. 



(b) Dissolve out the paraffin in one of the numerous sol- 

 vents (xylol, a few minutes). 



(c) At this point the xylol should be washed off with 

 absolute alcohol, and 



(d) The section is stained. 



