46 MATERIALS AND PROCESSES OF SLIDE-MAKING 



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 which one sees 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. 



Therefore the first step, in making a mount in Canada balsam, for instance, 

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

 second step is to make certain that one has natural Canada balsam and not 

 dried balsam which has been dissolved in xylene. Solutions of dried balsam 

 in hydrocarbons are meant for mounting 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. 10) 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, if one is using natural balsam 

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

 objects. Unfortunately most people are accustomed to mounting sections in 

 thin balsam by the technique shown in Fig. 11; that is, by touching one edge 

 of the coverslip to the drop and then lowering it from one side. The objec- 

 tion to this is that the balsam, as is seen in Fig. 11, 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. If one is mounting 

 specimens or deep objects in a cell in which a cavity has been ground, it is 

 desirable to hold the coverslip in place with a clip while the balsam is hard- 

 ening. This process is seen in Fig. 12; the type of clip there shown is made 

 of phosphor bronze wire and is far superior to any other type. 



The description above presumes that one is using natural Canada balsam, 

 unquestionably the best resinous medium in which to prepare wholemounts. 

 If one is using a solution of dried balsam in xylene, a very different technique 

 will have to be adopted. In the first place, most of these solutions are so thin 

 that it is^ almost impossible to apply the coverslip as shown in Fig. 10, and 

 one is forced to adopt the technique shown in Fig. 11. This difficulty may be 

 avoided by placing the object on the slide, putting a drop of the medium over 

 the object, and then setting the slide in a desiccator until most of the solvent 



