Making Smears and Squashes 121 



drop is placed about 1 in. from one end of the slide. A second slide, as 

 shown in Fig. 85, is placed in front of the drop. Capillary attraction 

 will distribute the fluid along the edge of the second slide, which is then 

 (Fig. 86) pushed sharply forward until it reaches the end of the bottom 

 slide. The material of which the smear is made is thus dragged out behind 

 the first slide and distributed more or less uniformly on the under slide. 

 A few people still try to conduct the operation in the reverse manner by 

 placing the second slide on top of the first, sloping it at a reverse angle to 

 that shown, and then endeavoring to push rather than drag the material 

 across the lower slide. The objection to this is that it results in crushing 

 cells, although it must be admitted that it frequently results in a more uni- 

 form distribution of the material. 



The method just described is the standard procedure for producing 

 "thin smears." These are necessary for those fluids, such as vertebrate 

 blood or mammalian seminal fluid, which contain large numbers of ob- 

 jects requiring wide separation for satisfactory study. 



There are a number of fluids, however, from which thick smears must 

 be made, either because they contain relatively few cells, as in the case of 

 invertebrate blood, or because they contain parasites that are distributed 

 relatively sparsely through the material, as in the case of malarial diag- 

 nostic smears. These thick smears are commonly made with the aid of a 

 loop of wire held in a needle holder of the type found in bacteriological 

 laboratories. This loop is dipped into the fluid to be examined. The mate- 

 rial is spread with a rotary motion in the center of the slide. This is very 

 similar to the preparation of smears of bacterial material, which is de- 

 scribed in some detail in Example 7. 



Fixing Smears. Smears may be fixed by drying, by alcohol, or in one of 

 the conventional fixatives. When a smear is to be fixed only by drying, it is 

 dried by waving it in the air, as soon as it has been made, and then set- 

 ting it aside for later treatment. This procedure is excellent in the case of 

 objects such as bacteria or erythrocytes, which do not change their shape 

 after drying; or for materials such as white blood corpuscles, which it is 

 not desired to preserve in their normal shape. No other object, however, 

 can be considered satisfactory unless it has been fixed. The simplest 

 method of doing this is to pass the smear, just as it is drying, through a 

 jet of steam. This technique has already been described for mounting 

 amebas and need not be repeated here. 



All other smears should be fixed before they are dried, and it is some- 

 thing of a problem to fix them without removing the material from the 

 slide. It is obvious that if the material is freshly smeared onto a glass slide 

 and then dropped into a fixative of some kind or another, it will be likely 

 to be washed off. The logical solution to the problem is to use a fixative in 

 a vapor phase, and nothing is better for this purpose than osmic acid. To 



