62 The Preparation of Microscope Slides 



Preserve in 70% alcohol 



Stain 



/ 



Dehydrate in alcohol 



i 

 Clear in oil 



i 

 Permanent mount in resin 



Smears. Smears, as the name indicates, are prepared by smearing a 

 fluid such as blood, or a bacterial culture, as a thin layer on a slide. This 

 layer is then stained to disclose its structure and air-dried. It may sub- 

 sequently be preserved in dry form or mounted in resin. In diagrammatic 

 form: 



Organic fluid 



I 

 Smeared on slide 



/ I 



Stain Stain 



i i 



Dry Dry 



I 

 Mount in resin 



Squashes. Organisms obviously should not be squashed if it is desired 

 to preserve the structure. The process is, however, invaluable when 

 it is desired to study the contents of a cell without regard to the shape or 

 relationships of the cell itself. In practice this method is usually confined 

 to the study of chromosomes. In diagrammatic form: 



Fresh tissue, either plant or animal 



i 

 Drop of stain 



' I 

 Squash under coverslip and examine 



Sections. Organisms too large to be made into wholemounts, but too 

 small to be studied by dissection, can conveniently be examined in the 

 form of thin slices or "sections." This is, moreover, the only way in which 

 the individual cells of organs can conveniently be displayed. Many plant 

 and most animal tissues are too soft to be cut into thin slices and must 

 therefore first be hardened. Reagents which fix the tissues in much the 

 same form as they had in life can be selected for this purpose. After 

 fixation the hardened tissues must be washed and they are then com- 

 monly preserved in 70 per cent alcohol. Most tissues, even after harden- 

 ing, must be supported during the process of cutting and to this end are 

 conventionally impregnated with wax. This process, just as the impreg- 



