Making Smears and Squashes 123 



an embryo flower or the salivary glands of a Drosophila larva may be 

 squashed in a drop of stain to facilitate the study of the chromosomes. 



Squashes are usually regarded as temporary preparations and are un- 

 doubtedly best used for this purpose. The difficulty in transforming them 

 into permanent mounts is that the removal of the coverslip, to permit de- 

 hydration and clearing, often disturbs or even removes the squash itself. 

 This can often be prevented by freezing. In this process the slide is laid 

 on a block of dry ice— the stage of a freezing microtome is less effective— 

 and left for about 5 min, or until the coverslip is thoroughly coated with 

 frost. A safety-razor blade can then be gently slid along the slide and 

 the coverslip levered off the preparation. The adhesion of the latter to 

 the slide through the subsequent processes of dehydration and clearing 

 is, of course, dependent on the nature of the preparation. 



