Making Wholemounts 115 



result of imperfect dehydration until after the specimen has been mounted 

 in balsam. 



The first choice is terpineol ("synthetic oil of lilac"), which has ad- 

 vantages possessed by no other oil. It is readily miscible with 90 per cent 

 alcohol, so that it will remove from the specimen any traces of water that 

 may remain in it through faulty dehydration, and it does not make speci- 

 mens brittle. It also has a very slight and rather pleasant odor. Clove oil 

 is the most widely recommended essential oil for the preparation of whole- 

 mounts. It has only two disadvantages: its violent odor and the fact that 

 objects placed in it are rendered brittle. If a small arthropod is cleared 

 in clove oil, it is almost impossible to get the animal into a wholemount 

 without breaking off some appendages. Clove oil, however, is miscible 

 with 90 per cent alcohol. 



Mounting in Balsam. Nothing is easier than to mount a specimen in 

 balsam, provided that it has been perfectly dehydrated and cleared. A 

 properly made wholemount should be glass-clear, but it will not be clear 

 in balsam unless it is clear in terpineol or clove oil. Not more than one in a 

 thousand wholemounts has this vitreous appearance. The worker who is 

 accustomed to looking at rather cloudy wholemounts should take the 

 trouble to dehydrate a specimen thoroughly, to remove the whole of the 

 dehydrating agent with a clearing agent, and then to mount the specimen 

 properly in balsam. 



The first step in making a mount in Canada balsam is therefore to make 

 quite certain that the specimen in its essential oil is glass-clear. The second 

 step is to use natural Canada balsam and not dried balsam dissolved in 

 xylene. Solutions of dried balsam in hydrocarbons are meant for mount- 

 ing sections and, for this purpose, are superior to the natural balsam. 

 Natural balsam is, however, just as preferable for wholemounts and is 

 just as easy to obtain. If it is found to be too thick for ready use, it may 

 be warmed gently to the desired consistency. A single small specimen is 

 mounted by placing it in a drop of balsam on a slide and then lowering 

 a coverslip horizontally ( Fig. 81 ) until the central portion touches the 

 drop. The coverslip is released and pressed very gently until it just 

 touches the top of the object. By this means it is possible to retain the 

 object in the center of the coverslip and also, when using natural balsam, 

 which does not shrink in drying, to avoid cells for any but the largest 

 objects. Unfortunately most people are accustomed to mounting sections 

 in thin balsam by the technique shown in Fig. 82; that is, by touching 

 one edge of the coverslip to the drop and then lowering it from one side. 

 The objection to this is that the balsam, as is seen in Fig. 82, immediately 

 runs into the angle of the coverslip, taking the object with it, and it is 

 difficult to lower the coverslip in such a way that the object is left in the 

 center. When mounting specimens or deep objects in a cell in which a 



