90 



THE ART OF MAKING MICROSCOPE SLIDES 



Fixation 



European elderberry {S. nigra). This dif- 

 ference between the two species may ac- 

 count for the disfavor in which elder pith 

 is held in the United States, but in the 

 writer's experience it is far more conven- 

 ient than the carrot. The disadvantage of 

 the carrot is that it must be absolutely 

 fresh, and even if it is kept in water over- 

 night it loses much of that crispness which 

 is necessary for the production of a good 

 section. 



Almost all hand sections are cut from 

 plant material, and most of them from 

 leaves or stems. To support a leaf, a 

 cylinder, of the right diameter to fit in the 

 microtome is cut either from elder pith or 

 carrot, split down the middle, and the leaf 

 inserted. The holding screw is then tight- 

 ened. Stems, however, cannot be held by 

 this means and a hollow cjdinder must be 

 prepared with an outer diameter con- 

 venient to the microtome and an inner 

 diameter which is slightlj' less than that 

 of the stem to be gripped. This hollow 

 cylinder is then split, the stem inserted, 

 and the section cut. Of course, a few sul)- 

 stances, such as cork or stiff plant stems, 

 may be cut without any other support; 

 these are, however, in the minority. 



Hardening and Fixing Materials for 

 Cutting 



Many objects, which are in themselves 

 unsuitable for sectioning by hand, may be 

 made more suitable if they are fixed and 

 hardened. A general discussion of the 

 principles governing the selection of a fixa- 

 tive is given in Chapters 6 and 12; the 

 formulas which have been suggested for 

 the purpose are given in Chapter 18. If, 

 however, one is to go to the trouble of 

 hardening and fixing material in a formula 

 designed to preserve the structure of the 

 cells, it is usually worth while to go to the 

 additional trouble of embedding the ma- 

 terial and cutting sections as described in 

 the next two chapters. For material to be 

 hand sectioned it is sufficient that it be 

 preserved in 90% alcohol. This process is 

 equally appUcable to the stems and leaves 

 of the botanists, or to the verj' few animal 

 materials, such as cartilage, which are suit- 

 able for the production of hand sections. 



It must not be thought that objects em- 

 bedded in nitrocellulose or wax should not, 

 or cannot, be cut on a hand microtome. 

 The study of the embryology of the frog 

 is rendered much easier to elementary 

 students if they are allowed to cut hand 

 sections of blastulae, gastrulae and young 

 larvae for themselves; and a word might 

 be said at this point as to the method by 

 which such blocks maj^ be readily pre- 

 pared in quantity. The larvae are fixed, 

 dehydrated, cleared, and impregnated 

 with wax in the manner described in Chap- 

 ter 12. Instead of casting each into an 

 individual block, however, a large slab of 

 wax — the cheapest paraffin is suitable — 

 is cast in a tray. A heated iron rod of 

 about I'^-inch diameter is then driven into 

 the slab so as to make a little pool of 

 molten wax. An impregnated larva, or 

 egg, is then dropped into the hole and the 

 process repeated. By this means a couple 

 of hundred frog eggs may be embedded in 

 ten minutes. The large block of wax, con- 

 taining numerous eggs, is then cut with a 

 saw into rectangles, the edges of which 

 are trimmed with a knife until they will fit 

 into a hand microtome. These blocks may 

 even be cut without a hand microtome by 

 placing the block on a bench and shaving 

 off successive sections with a sharp scalpel. 



Staining and Mounting Sections 



Sections, wliich are taken individually 

 from the knife and accumulated in a dish 

 of 70% alcohol, should be treated as 

 wholemounts rather than as sections. 

 They may, that is, be directly mounted in 

 either gum media (Chapter 4) or jelly 

 media (Chapter 5) or they may be stained 

 and mounted in resinous media in the 

 manner described in Chapter 6. 



It might be pointed out, however, that 

 many sections may be double- or triple- 

 stained (a process which is impossible with 

 wholemounts) and that in theory any 

 method of staining described in Chapters 

 20, 21, or 23 for wax or nitrocellulose sec- 

 tions may also be applied to a hand sec- 

 tion. These are, however, much better 

 applied to sections after they have been 

 attached to a slide. If they are to be at- 

 tempted, reference should be made to 



