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XLV] 65 μ by 32 μ to 43 μ; have a regular, somewhat elongated oval form, with a delicate, smooth, transparent shell, through which two, or four, or eight light-grey yolk segments can be distinctly seen. It is well to search for these ova soon after the fæces have been passed; otherwise, owing to the rapidity with which, in favourable circumstances, development proceeds, the embryo may have quitted the shell and the egg be no longer visible.

Synonyms.—Ascaris trichiura; Trichocephalus hominis; T. dispar.

Fig. 171.—Ovum of A. lumbricoides, x 250. (Photograph by Dr. J. Bell.)

Trichiuris trichiura, the whip-worm (Fig. 174), lives chiefly in the cæcum. In many countries it is present in more than half the population. It is believed to maintain its position by transfixing, pin-fashion, with its long slender neck, a superficial fold of the mucous membrane. Wichmann claims to have shown, by serial sections of the cæcum at sites where the parasites were fixed, that it is merely embedded in the mucus between the intestinal villi. According to Powell, the females very much preponderate, the proportion to males being as 466 to 1. Except that the practitioner should be familiar with the appearance of its eggs in the stool (Fig. 168, c), so that he may be able to distinguish them from those of ascaris, of ankylostomum, and of other parasites, the presence of T. trichiura is of no practical moment. So far as known, it