EMBEDDING METHODS 117 



and allow the alcohol and ether to evaporate until the celloidin has 

 become of the consistency of a rather soft Swiss cheese. Then the 

 tissue with a little celloidin around it can be cut out and pasted with 

 a little thick celloidin on the fiber block. After which it is best to 

 leave it under a bell-jar with an open dish of chloroform. In this 

 manner the most homogeneous embedding best adapted for section- 

 ing is obtained. However, it is not necessary in ordinary work to 

 adhere strictly to all of these finer details. 



Celloidin embedded material is sectioned on the microtome in 

 such a manner that the microtome knife strikes the tissue very ob- 

 liquely. Both the knife and tissues should be kept constantly wet with 

 80 per cent, alcohol. The cut sections must be kept in 80 per cent, 

 alcohol until they can be stained. 



PARAFFIN EMBEDDING METHOD/ 1. Dehydrate the tissue well 

 in absolute alcohol. 



2. Place for several hours in equal parts of absolute alcohol and 

 xylol. 



3. Place for several hours into pure xylol. 



4. Place for several hours into xylol saturated with soft paraffin. 



5. Place for several hours in melted paraffin kept in a suitable 

 paraffin oven at about 54 C. 



6. Change the melted paraffin once during this time. 



7. Prepare a little paper box or place two lead squares on a glass 

 plate, pour some fresh melted paraffin into the square and place 

 the tissue in it. 



8. As soon as the paraffin is superficially hard place the tissue with 

 the surrounding paraffin and leads into the refrigerator or into cold 

 water, so that they are cooled very rapidly. This rapid cooling gives 

 to the paraffin a homogeneous consistency which makes it cut better, 

 and much thinner sections can be prepared. When the tissue and 

 the paraffin have been thoroughly cooled, the excess paraffin is 

 trimmed off, leaving the tissue surrounded by a very small amount 

 of the embedding material. The embedded tissue is then mounted 

 with a little melted paraffin on a block of wood or vulcanized wood 

 fiber. After the mounting is firm, the tissue is ready to be sectioned 

 on the microtome. 



Sectioning. Paraffin embedded material is sectioned dry with 

 the knife at right angles to the block, and with a rapid stroke. Par- 

 affin sections that are to be stained for the demonstration of bacteria 

 should be very thin not over five micra and should lie very flat 

 on the slide. This is accomplished by taking them from the knife, 



1 A very rapid paraffin-embedding procedure is the acetone method, which has the following 

 steps : 



1. Place small tissue into best water-free pure acetone for one hour. 



2. Change acetone and leave another hour. 



3. Drop into melted paraffin and leave in paraffin over one-half hour. 



4. Change paraffin and leave another half-hour. The tissue is now ready to be blocked and 

 sectioned. 



