FROZEN SECTIONS 121 



and should be left about half an hour. Osmic-fixed tissues get 

 very brittle in the freezing technique. 



226. Cutting Sections. The piece to be cut should not ordinarily 

 exceed 3x3 cm. One corner should lie towards the knife, and 

 fibres should, if possible, be at right-angles to the edge of the 

 knife. No water, saline, gum, syrup, etc., should be brought 

 into contact with the tissue pieces. A little water will have to 

 be i:)ut on the microtome table freezer in order to freeze the 

 piece of tissue securely on to the table. The indicator for thick- 

 ness of sections should be placed at 20 /x or thereabouts, when the 

 beginner is practising — it is difficult to cut thinner sections. 



According as to whether the sections are to be stuck on to 

 slides with gelatin, etc., or " dry " by their own juices coagulated 

 by some suitable vapour, the procedure is slightly different. 

 The former method is probably the easier, and should be mastered 

 first. In both cases, however, the temperature of the frozen piece 

 is important and can only he judged by experience. Material like 

 brain, liver, spleen, etc., should be about — 10° to — 15° C. ; 

 fat, ligament, etc., — 20° to — 30° C. Turn on the cock on the 

 cylinder of COg, open the lever below the freezer, and with several 

 openings and shuttings of the pin valve of the freezer, the block 

 becomes frozen. The knife, which has previously been brought 

 into proper juxtaposition to the block, by raising or lowering the 

 freezer table, is now passed over the tissue till sections begin to 

 cut. Look at these sections — if they have fine cracks on them or 

 are brittle, the block is too cold, if they are torn, the block is not 

 cold enough. A piece of liver is excellent material for judging 

 this degree of freezing, which can only be learnt after some 

 practice. The proper degree of temperature is more easily 

 judged with the microtome fitted with a knife freezing attachment. 

 When cut, ordinary mammalian sections of fixed material may be 

 removed from the knife with a brush or forceps and transferred 

 to a dish of distilled water or 10 per cent, formol salt in which 

 they may remain until stuck on slides as described below. Sections 

 of such materials as mollusc ovotestis and mammalian testis 

 tend to break up if floated out in a dish, and are best cut fresh, if 

 possible, according to the method described in § 230. However, 

 they can be cut and stuck on a slide if brought direct to the 

 gelatin coated slide one at a time. 



227. The Gelatined Slides. These are prepared by smearing 

 gelatin on the slide in the same manner as a blood film. About 

 2 grm. of the best gelatin is broken up and dissolved in 100 c.c. 

 of water as follows : it is first left to soak in enougli distilled 

 water to cover it for about two or three hours and then the rest 

 of the 100 c.c. of distilled water is added and heated to 50° or 60° C. 

 Clean slides are coated with this fluid as for making blood smears. 

 The slides are then tilted up against the back of the bench till 



