Belonging to the section _Disporea_, that is, forming only two spores, each with four sporozoites. *Isospora bigemina*, Stiles, 1891. Syn.: “_Cytospermium villorum intestinalium canis et felis_,” Rivolta, 1874; “_Coccidium Rivolta_,” Grassi, 1882; _Coccidium bigeminum_, Stiles, 1891. This parasite lives in the intestinal villi of dogs, cats, and the polecat (_Mustela putorius_, L.). According to Stiles,[188] the oöcyst divides into two equal ellipsoidal portions or sporoblasts which become spores and then each forms four sporozoites. The oöcysts of this species vary from 22 µ to 40 µ in length and from 19 µ to 28 µ in breadth. Each spore is 10 µ to 18 µ long and contains four sporozoites. The parasites live and multiply, not only in the gut epithelium, but also in the connective tissue of the intestinal submucosa. Wasielewski has seen merozoites in the gut of the cat. [188] “Notes on Paras.,” No. II, _Journ. of Comp. Med. and Vet. Sci._, 1892, xiii, p. 517. _Isospora bigemina_ (fig. 75) appears to occur also in man, for Virchow published a case which was communicated to him by Kjellberg, and attributed the illness to this parasite.[189] Possibly also it would be more correct to ascribe the observation of Railliet and Lucet, which is mentioned under “Human Intestinal Coccidiosis,” p. 148, to this species, as the Coccidia in that case were distinguished by their diminutive size (length 15 µ, breadth 10 µ). The case communicated by Grunow may also possibly refer to _Isospora bigemina_.[190] Roundish or oval structures of 6 µ to 13 µ in diameter occurred in the mucous membrane of the gut and in the fæces of a case of enteritis. [189] _Arch. f. path. An._, 1860, xviii, p. 527. [190] Grunow, “Ein Fall von Protozoën (Coccidien?) Erkrankung des Darmes,” _Arch. f. exper. Path. und Pharm._, 1901, xlv, p. 262. [Illustration: FIG. 75.--_Isospora bigemina_, Stiles (from the intestine of a dog). _a_, Piece of an intestinal villus beset with Isospora, slightly enlarged; _b_, _Isospora bigemina_ (15 µ in diameter), shortly before division; _c_, divided; _d_, each portion encysted forming two spores; _e_, four sporozoites in each part, on the left seen in optical section, together with a residual body--highly magnified. (After Stiles.)] DOUBTFUL SPECIES.