Page:Tropical Diseases.djvu/865

Rh that is necessary, by way of preparation, is to place on the slip a minute portion of the suspected fæces—about the size of a hemp-seed—and then to apply the cover-glass, gently gliding it over the slip so as to spread out the mass in a thin, fairly uniform, and transparent layer. If the bit of fæces prove too consistent, a little water may be added so as to soften it. If the stools are loose and watery, the sediment should be taken up with a pipette and examined. It is well to make two or three preparations. The microscopical examination must, in the first instance, be made with a low power—an inch or, better, a half-inch objective. Search must be made in every part of the slide, and every suspicious body carefully scrutinized, a higher power being used if necessary. A very little practice suffices for the identification not only of the commoner ova, but of the species of parasite to which the ova belong.

The points to be attended to in the diagnosis of ova are size, shape, colour, thickness, roughness, smoothness, and markings on the surface of the shell; the presence or otherwise



of yolk spheres, of a differentiated embryo, or, in the case of the cestodes, of the three pairs of embryonic hooklets; the existence of an operculum in the case of certain trematodes and of the broad tapeworms (Dibothriocephalus). The ova of the same species of parasite vary but slightly, and are in every instance sufficiently stable and definite for correct diagnosis.

Ova of Trichiuris trichiura (Fig. 168, c).—Of the three common nematodes mentioned—Trichiuris trichiura, Ascaris lumbricoides, and Ankylostomum duodenale—the ova of the first are the most frequently met with. They occur sometimes in enormous numbers, as many as six or eight specimens being visible in one field of an inch objective. They form a rather striking object under the microscope (Fig. 170). They are oval, measuring 51 to 54 μ by 21 to 23 μ, the ends of the long axis of the oval being slightly pointed and tipped with a little shining projection or plug. Their general appearance suggests an elongated oval tray, the projections at the poles of the ovum representing the handles of the tray. They are