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810 dark brown in colour, sharply defined, doubly outlined, and contain no differentiated embryo.

The ova of Ascaris lumbricoides (Fig. 168, f, g, and Fig. 171) are considerably larger (60 to 75 μ by 40 to 58 μ) than those of trichiuris (Fig. 168, c). They are also, as a rule, more spherical, or rather, more broadly oval; occasionally they are almost barrel-shaped. Like those of trichocephalus, they are dark brown in colour from bile-staining, but they are much less sharply and smoothly defined, possessing a coarse thick shell which is roughened by many warty excrescences. The yolk contents are not always easily made out, nor, when made out, can any sign of embryo or segmentation be discovered.

Fig. 170.—Ovum of Trichiuris trichiura, x 280. (Photograph by Dr. J. Bell.)

In certain instances, supposed to be unfertilized, the ova are smooth on the surface, the rough outer layer being almost or altogether absent.

A point of practical importance to be attended to lies in the circumstance that the rough outer layer on the shell of the ovum of ascaris is very easily detached, leaving the egg with a sharp, smooth outline suggestive of some other species of parasite. To obviate this, in mounting fæces it is well to avoid too much gliding of the cover-glass over the slip.

The ova of Ankylostomum duodenale (Fig. 168, b) contrast very markedly with both the foregoing, particularly in the matter of colour. Trichocephalus and ascaris ova are invariably dark and bile-stained; those of the ankylostomum are beautifully clear and transparent; they measure 55 μ to