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XL] at a somewhat later period after feeding, it will be found that the stomach of the insect, though still full of blood, contains very few microfilariæ, although their empty sheaths can be seen in abundance. If, however, we break up with needles the thorax of the insect and tease out in normal salt solution the muscular tissue, we shall find that the microfilariæ, after discarding their sheaths, have



quitted the stomach and entered the thoracic muscles of the insect, among the fibres of which they are now moving languidly. (Figs. 102-8.) By a course of serial dissections of filariated mosquitoes we can ascertain that in the thorax of the insect the parasite enters on a metamorphosis which takes from twelve to twenty days (longer or shorter, according to atmospheric temperature) to complete—a metamorphosis eventuating in the formation of a mouth, of an alimentary canal, and of a peculiar trilobed caudal end, as well as in a relatively