92 MICROSCOPIC METHODS 



In the case of very small pieces of tissue the time given for eacli stage 

 may be much shortened, and where haste is desirable Nos. 2 and 4 may 

 be omitted. Otherwise it is better to carry out the process as de- 

 scribed. When it is advisable to avoid all shrinkage it is well to change 

 the paraffin every few hours during the embedding process. 



6. Cast the tissues in blocks of paraffin as follows : Pairs of L- shaped 

 pieces of metal made for the purpose by instrument makers must be at 

 hand. By laying two of these together on a glass plate, a rectangular 

 trough is formed. This is filled with melted paraffin taken from a stock 

 in a separate dish. In it is immersed the piece of tissue, which is lifted 

 out of its pure paraffin bath with heated forceps. The direction in 

 which it is to be cut must be noted before the paraffin becomes opaque. 

 When the paraffin has begun to set, the glass plate and trough have 

 cold water run over them. When the block is cold, the metal L's are 

 broken off, and, its edges having been pared, it is stored in a pill-box. 



The Cutting of Paraffin Sections. Sections must be cut as 

 thin as possible, the Cambridge rocking microtome being, on 

 the whole, most suitable. They should not exceed 8 /x, in thick- 



FIG. 43. Needle with square of paper on end for manipulating paraffin 

 sections. 



ness, and ought, if possible, to be about 4 p. For their mani- 

 pulation it is best to have two needles on handles, two camel's- 

 hair brushes on handles, and a needle with a rectangle of stiff 

 writing paper fixed on it as in the diagram (Fig. 43). When 

 cut, sections are floated on the surface of a beaker of water kept 

 at a temperature about 10 C. below the melting-point of the 

 paraffin. On the surface of the warm water they become perfectly 

 flat. 



Fixation on Ordinary Slides, (a) Gulland's Method. A. supply of 

 slides well cleaned being at hand, one of them is thrust obliquely "into 

 the water below the section, a corner of the section is fixed on it with a 

 needle and the slide withdrawn. The surplus of water being wiped off 

 with a cloth, the slide is placed on a support, with the section down- 

 wards, and allowed to remain on the top of the paraffin oven or in a 

 bacteriological incubator for from twelve to twenty-four hours. It will 

 then be sufficiently fixed on the slide to withstand all the manipulations 

 necessary during staining and mounting. 



(b) Fixation by Mann's Method. This has the advantage of being 

 more rapid than the previous one. A solution of albumin is prepared 

 by mixing the white of a fresh egg with ten parts of distilled water and 

 filtering. Slides are made perfectly clean with alcohol. One is dipped 

 into the solution and its edge is then drawn over one surface of another 

 slide so as to leave on it a thin film of albumin. This is repeated with 

 the others. As each is thus coated, it is leant, with the film down- 



