[36] To explain this discrepancy it is stated that the border of the undulating membrane can be detached in the form of an independent flagellum. But Parisi (1910) places such quadriflagellate forms in the sub-genus _Tetratrichomonas_, _Arch. f. Protistenk._, xix, p. 232. There is an undulating membrane (fig. 16) which runs spirally across the body, arising from the place of insertion of the flagella, and terminating at the base of the caudal process. A cytostome seldom is recognizable in fresh specimens, but is apparently present. The nucleus is vesicular, elliptical and situated near the anterior extremity.[37] [37] According to Marchand, the nucleus is connected with a line, which becomes visible on addition of acetic acid, terminates at the posterior extremity, and does not correspond to the line of insertion of the undulating membrane. This formation probably is the same as the axostyle in _Trichomonas batrachorum_, Perty. Blochmann (1884) also mentions two longitudinal rows of granules, which commence at the same place as the nucleus and converge posteriorly. Multiplication takes place by division (Marchand). Encysted forms are almost unknown. _Trichomonas vaginalis_ lives in the vaginal mucus of women of various ages, not in normal mucus, but in mucus of acid reaction. It is found in menstruating females as well as in females who have passed the menopause. It occurs in pregnant and non-pregnant women, even in very young girls, provided always that they have a vaginal catarrh with acid reaction of the secretion. Should the acid reaction change, as, for instance, during menstruation, the parasites disappear, as they do likewise on injection of any alkaline fluid into the vagina. A low temperature (below +15° C.) is also fatal to the parasites. These flagellates can pass from the vagina through the urethra into the bladder, and produce severe catarrh, and are not easily removed. [Illustration: FIG. 16.--_Trichomonas vaginalis_, Donné. × 2,000 approx. (After Künstler.) Four flagella are represented, but usually only three are present.] _T. vaginalis_ appeared to be a parasite specific to the female organs and not transmissible to man. However, several observations have since been made that confirm the occurrence of this species in the urethra of the male. The infection apparently takes place through coitus when changes are present in the urethral mucous membrane. At any rate, three cases observed point to this circumstance. Attempts at experimental transmission to rabbits, guinea-pigs and dogs failed (Blochmann, Dock). So far, the manner in which women become infected is unknown. *Trichomonas intestinalis*, R. Leuckart, 1879 = *Trichomonas hominis*, Davaine, 1854.