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XL] An inch or half-inch objective and a mechanical stage with a parallel movement will enable the investigator to pass rapidly in review the whole of the blood on the slide. In unfixed slides, if microfilariæ are present, they will be detected readily; the hæmoglobin of the blood corpuscles being dissolved out by the watery stain, the nuclei of the white blood-corpuscles and any microfilariæ that may be present are the only coloured objects visible on the slide, and therefore at once catch the eye.

In any district in which the filaria is moderately common, out of 100 slides prepared in this way from as many individuals, probably eight or ten will be found to contain the parasite. When microfilariæ have been detected, the persons from whom the parasite-bearing slides came may be used afterwards as a source of supply for further examinations and study.

Demonstration of living microfilariæ.—When it is desired to study the living microfilaria, all that is necessary is to make three or four ordinary wet preparations of the blood of a filaria-infected person—making them during the evening or night, and ringing the cover-glasses with vaseline so as to prevent the slides from becoming dry. In such preparations, if kept cool, the microfilariæ remain alive for a week, or even longer, and can readily be detected by their movements, an inch or half-inch objective being used in the first instance as a searcher.

Permanent preparations.—Permanent preparations may be made by fixing very thin films of blood with alcohol or heat, staining with methylene blue, eosin, etc., and mounting in xylol balsam. It is generally advisable before fixing to wash out the hæmoglobin with water or very weak acetic acid. Logwood is perhaps the best stain; it brings out the sheath very distinctly, and picks out the nuclei. Double staining with eosin and logwood shows very well the structure of the musculo-cutaneous layer of the worm, in addition to other anatomical details.

Description of larval form.—Examined in fresh blood, microfilaria bancrofti (Fig. 97) is seen to be a minute, transparent, colourless, snake-like organism, which, without materially changing its position on the slide, wriggles about in a state of great activity, constantly agitating and displacing the corpuscles in its neighbourhood. At first the movements are so active that the anatomical features of the microfilaria cannot be made out. In the course of a few hours the movement slows down, and then one can see that the little worm is shaped like a snake or an eel; that is to say, that it is a long, slender, cylindrical organism, having one extremity abruptly rounded off, and the other for about one-fifth of the entire length gradually tapered to a fine point. On measurement, it is found to be a little over or under 0·3 mm. in length by 0·008-0·011 mm. in diameter—about the diameter of a red blood-corpuscle.

When examined with a low power, it appears to be structureless; with a high power a certain amount of structure can, on