MAKING WHOLEMOUNTS 47 



has evaporated. A second layer is placed on top to build up a large drop or, 

 rather, a thick coat of varnish over the specimen. A coverslip is applied, and 

 the slide is warmed until the resin becomes fluid. 



The best use for solutions of balsam in making wholemounts is in dealing 

 with a very large number of objects. The method in this case is to transfer 

 the objects from their clearing medium to a tube or dish of the solution of 

 balsam in whatever hydrocarbon has been selected and then permit the hydro- 

 carbon to evaporate. When the balsam which remains has reached a good 

 consistency for mounting, take each specimen, together with a drop of balsam, 

 place it on a slide, and add a coverslip. By this method large numbers of slides 

 can be made in a short time. It is not necessary to use solutions of dried balsam, 

 and the author prefers, for this purpose, to dilute natural balsam with benzene. 

 Mounting large objects in a deep cell in Canada balsam is not to be recom- 

 mended because the balsam becomes yellow with age and, when in thick layers, 

 tends to obscure the specimen. A wholemount of a 96-hour chicken embryo, 

 for example, is of extremely doubtful value, but if it has to be made it is best 

 first to impregnate it thoroughly with a fairly thin dilution of natural balsam. 

 It is then placed in the cell, piling the solution up on top, and left in a desic- 

 cator. The cell is refilled as the evaporation diminishes the contents; when 

 completely filled with solvent-free balsam, it is warmed on a hot table. The 

 coverslip is applied directly. 



Finishing Balsam Mounts. If a mount has been made correctly with 

 natural balsam, and if the size of the drop has been estimated correctly, no 

 finishing is required since no balsam will overflow the edges of the coverslip. 

 Natural balsam is very thick when cold, so that the coverslip will not become 

 displaced if the mount is handled before the balsam is fully hard. The hard- 

 ening may be aided by heat, but one must be carefiil in heating thick mounts, 

 particularly where the coverslip is not supported by a cell, that the liquefaction 

 of the balsam does not cause the coverslip to tip sideways. Despite the fact 

 that drying time is sometimes prolonged, natural balsam should always be 

 used for these thick mounts because, if a solution of dried balsam and xylene 

 is employed, the evaporation of the solution will cause huge air bubbles to be 

 drawn under the coverslip. When it is sufficiently hard, the slide should be 

 cleaned, first, by chipping off^ any excess balsam with a knife and, secondly, 

 by wiping away any chips with a rag moistened in 90 per cent alcohol. This 

 will leave a whitish film over the surface of the slide, which may then be re- 

 moved with a warm soap solution. The slide should be polished before 

 being labeled. 



With regard to labeling, it may be pointed out that no power on earth will 



