134 The Preparation of Microscope Slides 



ing agent, and that the object is in a vial containing a few milliliters of 

 this solvent. Take a block of whatever medium is to be used for em- 

 bedding and shave a few chips from it with a knife. Add these to the 

 solvent. The chips usually dissolve very slowly and form a thickened 

 layer at the bottom of the tube through which the object to be embedded 

 sinks. The average object will be satisfactory if left overnight. Then place 

 the tube itself in the embedding oven maintained at a temperature some- 

 what above that of the melting point of the wax and add as many more 

 shavings as can be crammed into the tube. When these are completely 

 melted and a large quantity of the volatile solvent has evaporated, remove 

 the object with a pipette or forceps and place it in a dish of pure wax for 

 an hour or two before transferring it to a second dish of pure wax for 

 the time necessary to secure complete impregnation. 



There is no method of forecasting how long an object will take to be- 

 come completely impregnated with wax. It is very easy to find out (see 

 Table 2) when one has started to cut sections whether or not the im- 

 pregnation is complete, but there is no basis save experience on which to 

 establish the timing in the different baths of wax. If the object is to be 

 transferred directly from solvent to wax, at least three baths should be 

 employed, since nothing is more destructive to a good section than the 

 presence of a small quantity of the clearing agent in the embedding 

 medium. To an absolute beginner seeking a rough guess, it may be said 

 that a block of liver tissue, 3 to 5 mm in size, will be satisfactorily im- 

 pregnated with wax after 30 min in each of three baths, whereas a 96-hr 

 chick embryo will require at least 2 hr in each of three baths for its 

 successful impregnation. 



While the object is being impregnated with the wax, it is necessary to 

 decide what type of vessel will be used to cast the final block. This will 

 depend to a far greater extent on the size of the object than on the prefer- 

 ence of the worker. Very small objects may be embedded most satisfac- 

 torily in ordinary watch glasses ( that is to say, ordinary thin-walled watch 

 glasses, not Syracuse watch glasses of the laboratory type) or in any 

 other thin-walled glass vessel. Very large objects are often embedded 



Length of box + twice height of box+ twice length of flaps 



a> 



o 



Fig. 93. Laying out paper for an embedding box. 



