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XL] runs for some distance along the axis of the worm. Further, if a high power be used, a closely set, very delicate transverse striation can be detected in the musculo-cutaneous layer throughout the entire length of the animal. Besides this, if carefully looked for at a point about one-fifth of the entire length of the organism backwards from the head end, a shining, triangular V-shaped patch (Fig. 97, c) is always visible. What may be this V spot is brought out by very light staining with dilute logwood. The dye brings out yet another spot (Fig. 97, d), similar to the preceding, though very much smaller; this second spot is situated a short distance from the end of the tail. The former I have designated the "V spot," the latter the "tail spot." These spots are probably connected with development, the V spot being the rudiment of the future water-vascular system, the tail spot that of the anus or cloaca and posterior part of the alimentary canal. The spots (Fig. 97, c, d) are not stained by strong logwood or by the aniline dyes. Staining with logwood also shows that the body of the little animal is principally composed of a column of closely packed, exceedingly minute cells enclosed in a transversely striated musculo-cutaneous cylinder (Fig. 97); at all events, many nuclei are thereby rendered visible. Low has pointed out to me that the break seen in all stained specimens in the central column of nuclei occurs at a point slightly anterior to the V spot. This break can only be recognized in stained preparations.

Rodenwaldt and Fülleborn, by careful study of specially stained specimens, have further elucidated the histology of the microfilaria, which is fully illustrated in Figs. 98 and 128 borrowed from Fülleborn, special attention being directed to the excretory and genital cells and substances in the rudimentary intestinal canal. (See also Plate XI.)

When the movements of the living microfilariæ have almost ceased, by careful focusing it can be seen that the head end is constantly being covered and uncovered by a six-lipped—or hooked—and very delicate prepuce; and, moreover, one can sometimes see a short fang of extreme tenuity, based apparently on a highly retractile granule, suddenly shot out from the uncovered extreme cephalic end, and as suddenly retracted (Fig. 99, c, d).

Filarial periodicity.—A singular feature in the life of the microfilaria is what is known as "filarial periodicity."