62 



THE ART OF MAKING MICROSCOPE SLIDES 



Pectinatella 



ferentiating solution, a single specimen 

 should be transferred to a watch glass and 

 examined under a low power of the micro- 

 scope. It is more than probable that little 

 differentiation will be required, so that a 

 simple rinse may be adequate. It is difficult 

 to judge the exact degree of differentia- 

 tion, but it must be remembered that the 

 object will appear darker after clearing 

 than it does in the differentiating solution. 

 The internal organs should be sharply de- 

 marcated when the outer surface of the 

 specimen is relatively free from stain. This 

 may be judged in Pectinatella by placing a 

 covershp on the specimen and examining 

 one of the branches of the lophophore 

 under the high power of the microscope. 

 Differentiation may be considered com- 

 plete when only the nuclei in the cells of 

 the lophophore are stained. The speci- 

 mens are then washed in four or five 

 changes of 70% alcohol, to remove the 

 strontium chloride, before being placed for 

 at least a day in 96% alcohol as the first 

 stage of dehydration. They should then 

 be transferred to two changes, with at 

 least six hours in each, to a considerable 

 volume of fresh 95% alcohol and may 

 then be cleared. Absolute alcohol is not 

 necessary if terpineol is the clearing agent. 

 There is some danger, if the specimens 

 are transferred directly from 95% alcohol 

 to a fluid as viscous as terpineol, that the 

 specimens will become distorted through 

 the very violent diffusion current. This 

 may be avoided in the following manner: 

 one takes a fairly wide (about an inch) 

 glass vial and fills it about half full of 

 terpineol. Ninety-five per cent alcohol is 

 then carefully poured down the side of the 

 vial (or on to a spoon held in the vial in 

 the manner of a bartender making a 

 pousse-cafe) so as to float a layer of alco- 

 hol on top of the terpineol. The specimens 

 are now dropped into the alcohol and sink 

 through it, coming to rest on the surface 

 of the layer of terpineol into which they 

 sink slowly without any strong diffusion 

 currents. They will have sunk to the 



bottom after a little while, but there will 

 still be alcohol diffusing upward from 

 them. As soon as the diffusion currents 

 have ceased, the alcohol should be drawn 

 from the top of the tube with a pipet and 

 the specimens transferred to clean ter- 

 pineol. When they are in fresh terpineol, 

 they should be examined carefully under 

 a microscope to make sure that they are 

 glass-clear without the least trace of milki- 

 ness. If they appear shghtly milky they 

 have either been insufficiently dehydrated, 

 or the alcohol used for the dehydration has 

 become contaminated with water. In 

 either case they must be transferred to a 

 tube of fresh alcohol for complete dehy- 

 dration and then put back into terpineol. 

 It is a waste of time to endeavor to pre- 

 pare a balsam mount from a specimen 

 which is not perfectly transparent in the 

 clearing medium. 



When all the specimens are in terpin- 

 eol, take some clean shdes, some clean 

 %-inch circular coverslips, and a balsam 

 bottle containing natural balsam. (The 

 author's preference for the natural balsam 

 rather than a solution of this material in 

 some solvent has already been explained.) 

 Place a drop of the natural balsam on each 

 of, say, six slides, and then one at a time 

 lift out six specimens from the terpineol 

 and place them on top of the balsam. They 

 will sink through the balsam slowly so that 

 these six slides should be pushed on one 

 side while a further six slides have drops 

 of balsam put on them, and so on. As 

 soon as the specimens have sunk to the 

 bottom of the drop of balsam, a covershp 

 is held horizontally above, touched to the 

 top of the drop, and then pushed down 

 with a needle until the specimen is flat- 

 tened firmly against the slide. As these 

 specimens have been properly hardened 

 and flattened there is no risk of their being 

 damaged by drying the mount under pres- 

 sure; therefore one can then apply a clip 

 (see Fig. 26) and place the shdes on a 

 warm table to harden. Each slide is then 

 cleaned, finished, and labeled as usual. 



Preparation of a Skeleton of an Insect in Balsam 



Two methods have already been de- gum media given in Chapter 4, and the 

 scribed for preparing small insects as simple, though very little-known, method 

 wholemounts. These are the use of the of dropping the insect directly into glacial 



