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

[Introduction]

The value of plastic embedded specimens as a teaching aid is now well recognised. It was to take advantage of these benefits, especially as a replacement for wet-mount bulky specimens, that a programme of embedding was initiated in the Zoology Department, Victoria University. From instruction leaflets, etc., the technique appeared to be a simple one, with little or no problems to trap the unwary. This we found to be true to a limited extent for the range of teaching material required in the department.

The teaching material we wished to embed covered a wide range both in surface and bulk texture and in size. The material varied from hard, barely porous shells and teeth to the open meshwork of the Porifera, through bone and cartilage to chitin-covered arthropods or coelenterates, and to delicate jellyfish or the early stages of the chick blastodisc. The largest specimen we have so far embedded is an eviscerated mammalian embryo 15 × 5 × 3.5 cm in which the skeleton had been stained with Alizarin Red S to distinguish the ossified tissue. The unpolished plastic block for this specimen measured 23 × 5 × 10 cm. The smallest individual specimens embedded were unstained copepods, 3-4 mm in length.

In the course of overcoming our difficulties with the embedding of such material, we also tried and found successful other procedures for sectioning the embedded material, and for block shaping and polishing. Comments on these procedures and on the embedding techniques we finally adopted we felt might be of interest and assistance to other workers contemplating plastic embedding of a similar range of zoological material. The quantities of material and the timing given for the embedding processes apply to polyester resin Au 8018C, styrine monomer and catalyst M.E.P.K., as manufactured by A. C. Hatrick (N.Z.) Ltd., Tawa, Wellington.

Basically, the chemicals and apparatus essential for embedding are the same if the embedding programme is extensive (for example, the skeletal material for an undergraduate class of some 150 students) or one of a few specimens for specialist teaching, or just general interest. But if the embedding programme to be undertaken is extensive, it is of value, particularly in terms of time saved and material used, if embedding moulds are built to size and can be used over and over again. The moulds shown in Figures 1, 2 and 4 also provide for easy release of the plastic block. Figure 4 illustrates the type of mould now most frequently used in the Department. Figures 1 and 2 show the type used in the experimental stage of our embedding programme. These latter moulds are well suited for a run of embedding requiring only a small number of specimens, say twenty or thirty. The pattern illustrated in Figure 1 is quick, easy and economical to make. We cut to shape formica topped bread boards, but "off-cuts" of any similar material obtainable from paint and glassware merchants would be equally satisfactory. Long wood screws fasten the two parts of the angle L-pieces together. The paired L-pieces of the mould can be adjusted to give a variety of shapes and sizes. However to obtain this flexibility of usage for a single pair of L-pieces, the joints and fit of the surface of the angles to the base plate must be leak page 2
Figs. 1 & 2 Formica faced L-pieces placed to form a mould.Fig. 3 Sectional view of mould to show the approximate degree of "tackiness of the resin surface prior to the pouring of the next layer.Fig. 4 Metal-sided, 4 in one mould showing also the glass cover plate.

Figs. 1 & 2 Formica faced L-pieces placed to form a mould.
Fig. 3 Sectional view of mould to show the approximate degree of "tackiness of the resin surface prior to the pouring of the next layer.
Fig. 4 Metal-sided, 4 in one mould showing also the glass cover plate.

page 3 proof. We attained this inexpensively by using plasticine and/or silicone wax in the manner illustrated in Figure 1. A variant of this type of mould we have found very useful for small specimens is one in which the wooden L-pieces are faced with glass instead of formica. We utilized 3" × 1" and 3" × 2" microscope slides for this purpose, and glued them to a wooden frame.

Flexibility of usage from a single "box" mould for large projects (Figure 4) is obtained by the provision of metal inserts, giving a one, two, three, or four-in-one box. Ease of removal of the blocks from this type of mould comes from the ability to open out the sides. The glass base and top plate give a surface to the hardened block that needs very little polishing. Virtually no presurfacing of the glass or metal with a releaser compound such as silicone wax is necessary as the plastic shrinks away from the mould during hardening.

Basically, the whole process of embedding can be summarized in four words: dehydrate, clear, embed the specimen and polish the plastic block. But before proceeding with a project, if you wish to admire your handiwork as a plastic block which has a high surface gloss, is almost crystal clear, and without bubbles, it is necessary to possess considerable patience and be aware of some of the problems that may be encountered in carrying out the four basic techniques noted above.