MAKING WHOLEMOUNTS 41 



vided for the coverslip the latter is bound to tip to one side or the other and 

 thus make the mount relatively useless. 



A good method of supporting a coverslip over a large object is to secure 

 some sheets of celluloid of various thicknesses and cut these into little squares 

 of about 1 mm. on a side. A drop of the mountant is placed on a slide, the object 

 placed in the mountant, and three of these little squares spaced equally around 

 the edge of it before the coverslip is applied. These squares may be made of 

 any desired thickness, so that one may accommodate objects of varying size. 

 This method, however, is not very satisfactory for objects which are much 

 more than 0.5 mm. thick, since the wide rim left at the edge of the cover causes 

 the mountant to dry out very rapidly. One is forced continually to add additional 

 mountant from the edges. 



The alternative is to provide a little box, known as a "cell," and to cement 

 this to the slide before mounting the object in it. Cells are available in a great 

 variety of materials, and the method of their use is described in some detail in 

 Gray's Microtechnique. For elementary purposes the best cells which can be 

 used are undoubtedly stamped from sheet tin or pewter and may be obtained 

 in thicknesses varying from V2 to about 1 Vi mm. They should be cemented to 

 the slide with some cement which is not soluble in balsam. The best general- 

 purpose adhesive is "gold size"; reference should be made to the source cited 

 above for details of this material and the method of its application. Glass cells 

 are available from biological supply houses. Although they look very attrac- 

 tive, there is no real advantage in their use since the object is usually studied 

 by transmitted light. 



Mounting in Resinous Media. Resinous media are used for wholemounts 

 not only because they permit the mounting of stained objects but more 

 particularly because they impart to the specimen a great degree of trans- 

 parency. This transparency comes from the increase in the index of refraction 

 of the specimen when it is completely impregnated with the resin. These 

 resins, however, are not miscible with water, so that one is forced first to remove 

 the water (dehydration) and then to replace the dehydrant with some material 

 (clearing agent) with which the resin itself is miscible. Before these operations 

 are conducted, the specimen must be killed and hardened (fixed), and it is 

 customary to stain the specimen in order to bring out those internal structures 

 which would become invisible, were they not colored, through the increase in 

 transparency. All of the following operations, therefore, must be executed and 

 will be discussed in turn. 



1. Narcotizing and fixing. 



2. Staining. 



