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Zoology Publications from Victoria University of Wellington—Nos. 42 to 46

Dehydration

Dehydration

A. Dry material

It is very desirable to know something of the past history of the material to be embedded. This is particularly the case with specimens that have been stored dry such as insects, crabs, spiders, corals, teeth, etc., but may have been subjected to fixation and/or temporary storage in liquid. If the past history of the specimen is not known, dry material is best placed in acetone under vacuum until all the air is removed, and the specimen thoroughly penetrated. If the specimen is large, this may take upwards of an hour utilizing vacuum pressure at 20-minute intervals. The specimen can then be redried or placed in acetone for storage prior to dipping it in the styrine monomer at the next stage in the embedding procedure (see also p. 4 for arthropod material).

B. Wet preserved specimens

The material should be thoroughly washed free of preservative and then dehydrated through 50% to 100% acetone. The length of time in which the specimen remains in the two grades of acetone depends, as with alcoholic dehydration, on the size and density of the specimen. In general, however, specimens of a similar size need less time in an acetone dehydration series than in an alcohol series. Acetone has another advantage over alcohol for dehydration in that it retains the colour of a wider range of specimens, particularly arthropods, than does alcohol. Furthermore, it does not matter for plastic embedding if the specimen is considerably hardened in texture by the acetone. In fact it is advantageous with soft bodied material as such material is then better able to withstand the pressure and heat of the setting resin without distortion.

This two-stage dehydration series proved quite satisfactory for all the material we have so far embedded, with two exceptions. These exceptions are echinoderms, and material that has been macerated in potassium hydroxide. With echinoderm material, a test specimen should first be tried in acetone, as acetone decolourizes and to some extent decalcifies many echinoderms. We have found it better to fix in formalin, wash in water, and then oven dry the specimen. Specimens that have had the tissues page 4macerated in potassium hydroxide need a more gradual dehydration than is afforded by the two-stage series. An 80% acetone bath should also be included in the series. Moreover, it will already have been appreciated that there will also be considerable shrinkage of this material. However, we found that both the monomer and the un-catalyzed resin restore in large measure the lost tonicity of the tissues.

It is also very advisable during dehydration to remove any trapped air in the specimen. This procedure is necessary particularly for material with an open mesh-work support, such as that found in the Porifera.

Any further preparation of the material prior to embedding now depends on the morphological features required for display. For example, with arthropods it is usually the exoskeletal features, but with soft-bodied animals it is more often the internal morphology. With the completion of dehydration in acetone, arthropods in which the exoskeletal features are required need only be dipped for a few minutes in the styrine monomer and they are ready for embedding in the catalyzed resin. Soft-bodied animals and any arthropods required for observation of internal morphology should be cleared to at least semi-transparency.