122 The Preparation of Microscope Slides 



use this material, take a petri dish and place in it a couple of thin glass 

 rods sufficiently far apart to permit the slide to rest on them without the 

 smear touching them. Then place a drop or two of a solution of osmic 

 acid, usually of 2 per cent strength, in the bottom of the petri dish and 

 replace the cover. It must be emphasized that osmic acid fixes the mucou s 

 membrane of the nose and throat just as readily as it does a smear, and 

 every caution must be taken to avoid inhaling the vapors of this mate- 

 ria l. As soon as the smear is made and before it has time to dry, it is placed 

 face down across the two glass rods, so that it is exposed to the vapor 

 but not to the liquid. The cover is then replaced on the petri dish, and the 

 slide left in place for 3 to 4 min, in the case of a thin smear, or 5 to 10 

 min in the case of a thick one. Then it is transferred to distilled water to 

 await staining. 



It occasionally happens that one must fix a slide in one of the conven- 

 tional fluid fixatives. This is done with the same petri dish and glass rod 

 setup as is used for vapor fixation, but in this instance the fixative is 

 carefully poured into the petri dish, which must be level, until it has 

 reached such a depth that, when the slide is laid across the glass rods, 

 the underside of the slide with the smear on it is in contact with the 

 fluid while the upper part is free from fluid. If the smear is reasonably 

 thin and is laid carefully in place, it usually will not become detached. 



Staining Smears. Blood smears are stained so universally with one or 

 another of the methylene blue-eosinate mixtures that it comes as 

 something of a surprise to most people to learn that any stain that is 

 suitable for sections may also be employed for smears. The advantage 

 of these mixtures for blood films is that the solvent methyl alcohol acts 

 as a fixative, so that the films, in effect, are stained and fixed in the same 

 operation. Where a blood smear is to be used for diagnostic purposes, 

 these techniques are excellent, since the appearance of the various types 

 of white corpuscles under this treatment is known to every technician. For 

 materials other than blood, there is no limit to the type of staining that 

 may be employed, although it must be remembered that these very thin 

 films require a stain of considerable intensity if the finer structures are to 

 be seen. 



SQUASHES 



Squashes are exactly what the name indicates and no theoretical dis- 

 cussion of this process is necessary. The object is placed on a slide, 

 crushed under a coverslip, and examined. 



The choice of the fluid in which it is crushed depends on the nature 

 of the object and the purpose of the preparation. Hydra, for example, may 

 be crushed in water— although cold-blooded Ringer's solution (p. 207) 

 is better— for the purpose of displaying the nematocysts. The anthers of 



