Varying views have been held as to the minute agency concerned in the production of this lesion. The bacillus discovered and studied by Ducrey, in 1889, is now accepted as the exciting cause. This is 1.5 μ long and 1 μ thick, with rounded deep-staining ends and fainter-staining central portion, occurring with great constancy in chancroidal pus, less often in buboes than in ulcers, in and outside of the cells, and in chains. It is cultivated with difficulty, grows best on human blood, takes basic aniline stains, but is easily decolorized by alcohol or by Gram’s method. Characteristic ulcers can be produced by inoculating it, even in monkeys. Chancroid begins, in twenty-four hours, as a red point or papule, which is quickly converted into a pustule and then into an _ulcer_. The borders of this ulcer enlarge, its depth increases, until after a few days it forms a more or less deep, often undermined excavation, irregular in contour, discharging grayish purulent material. In this respect it differs also from chancre, whose natural discharge is more like serum. In other words, _chancroid is essentially destructive, chancre constructive_, since the latter forms a new-growth which ordinarily has little or no discharge. When the necrosis of chancroid becomes extensive and tends to spread rapidly the ulcer is spoken of as _phagedenic_. This tendency to rapid local gangrene is the combined result, probably, of virulence of virus and lowered local or constitutional tissue resistance. It is consequently most often seen in alcoholics and prostitutes. In rare instances a surface larger than the hand may be rapidly destroyed, every particle of material sloughed being infectious. In chancroids of the mild variety the discharge may dry upon their surfaces and scabs or crusts result, beneath which, when detached, the characteristic ulcer is present. Under proper treatment this foul ulcer is soon converted into an ordinary granulating surface, which heals by cicatrization. CHANCROIDAL BUBO. Infection, by propagation along the lymphatics, of the inguinal nodes is frequent, and, since the infection is almost always a mixed one, suppuration is frequent. The pus of a suppurating chancroidal bubo is as infectious as the discharge from the original sore; hence the need of great caution. The edges of the local incision should be promptly cauterized so that they may not become linear chancroids. Phagedena shows itself here as well as about the genitals proper, and differs only in that it makes the case more serious. A chancroidal bubo may, however, subside without abscess formation. The signs of suppuration are those incident to pus formation anywhere near the surface. When pus is present its early evacuation is demanded.