Mounting 



WHOLEMOUNTS IN RESINOUS MEDIA 



57 



meant for mounting sections and are, for 

 this purpose, superior to the natural bal- 

 sam. Natural balsam is, however, just as 

 preferable for wiiolemounts and is just as 

 easy to obtain. If it is found to be too 

 thick for ready use, it may be warmed 

 gently until it reaches the desired consist- 

 ency. A single small specimen is mounted 

 by placing it in a drop of balsam on a slide 

 and then lowering a covershp horizontally 

 (Fig. 24) until the central portion touches 

 the drop. The coverslip is then released 

 and pressed very gently until it just 

 touches the top of the oliject. By this 

 means it is possible to retain the object in 

 the center of the covershp and also, if one 

 is using natural balsam which does not 

 shrink much in drjing, to avoid using cells 

 for any but the largest object. Unfortu- 

 nately most people are accustomed to 

 mounting sections in thin balsam by the 

 teclmique shown in Fig. 25: that is, b}^ 

 touching one edge of the covershp to the 

 drop and then lowering it from one side. 

 The objection to this is that the balsam, 

 as is seen in the figure, immediately runs 

 into the angle of the coverslip, taking the 

 object with it, and it is difficult to lower 

 the covershp in such a waj' that the object 

 is left in the center. If one is mounting thin 

 objects, 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 hardening. This proc- 

 ess is seen in Fig. 25, the type of chp there 

 shown being made of Phosphor bronze 

 wire, and is far superior, in the writer's 

 opinion, to any other type. 



This description presumes that one is 

 using natural Canada balsam, which is un- 

 questionably the best resinous medium in 

 which to prepare wholemounts. If one is 

 using one of the thin resinous media, many 

 formulas for which are given in Chapter 

 26 under the heading M 30, a very differ- 

 ent technique wiU have to be adopted. In 

 the first place these media are so thin that 

 it is almost impossible to ajiply the cover- 

 shp as shown in Fig. 24, and one is forced 

 to adopt the technique shown in Fig. 25. 

 This difficulty may be avoided by placing 

 the object on the slide, placing a drop of 

 the medium over the object, and tlicn 

 placing the shde in a desiccator until most 



of the solvent has evaporated. A second 

 layer is then placed on toj-), and a large 

 drop, or rather a thick coat of varnish, is 

 thus built up over the specimen. A cover- 

 slip is then applied and the slide warmed 

 until the resin becomes fluid. 



The best use for solutions of balsam in 

 making wholemounts is in the preparation 

 of dehcate specimens or a large number of 

 objects. The technique for the former is 

 described in Chapter 20. In the latter case 

 the objects are transferred from the clear- 

 ing medium to a tube or dish of the solu- 

 tion of balsam in whatever hydrocarbon 

 has been selected, and the solvent then 

 evaporated. When the balsam which re- 

 mains has reached a good consistency for 

 mounting, each specimen is taken, to- 

 gether with a drop of balsam, and placed 

 on a slide. A covershp is then added. By 

 this method large numbers of shdes may 

 be made in a short time. It is not necessary 

 to use solutions of dried balsam, and the 

 writer prefers, for this pur^DOse, to dilute 

 natural balsam with benzene. 



Mounting large objects in a deep cell in 

 Canada balsam is not to be recommended 

 for the reason that the balsam becomes 

 yellow with age and, in thick layers, tends 

 to obscure the specimen. A wholemount of 

 a 96-hour chicken embryo, for example, is 

 of very 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 desiccator. The cell is refilled as the 

 evaporation of the solvent lowers the level. 

 When the cell is finally completely filled 

 with solvent-free balsam it is warmed 

 on a hot table and the covershp applied 

 directly. 



Finishing Balsam Mounts 



If a mount has been properly made with 

 natural balsam, and if the size of the drop 

 has been estimated correctly, no finishing 

 is recjuii'cd since no balsam will overflow 

 the edges of the coveislij). Natural balsam 

 takes a long time to harden and, if one has 

 a fairly thick mount the covershp of 

 which is not supported, drying cannot be 

 hastened by heating as this will liciuify 

 the balsam, causing the coverslip to tip 



