#Primary Syphilis.#--_The period of incubation_ represents the interval that elapses between the occurrence of infection and the appearance of the primary lesion at the site of inoculation. Its limits may be stated as varying from two to six weeks, with an average of from twenty-one to twenty-eight days. While the disease is incubating, there is nothing to show that infection has occurred. _The Primary Lesion._--The incubation period having elapsed, there appears at the site of inoculation a circumscribed area of infiltration which represents the reaction of the tissues to the entrance of the virus. The first appearance is that of a sharply defined papule, rarely larger than a split pea. Its surface is at first smooth and shiny, but as necrosis of the tissue elements takes place in the centre, it becomes concave, and in many cases the epithelium is shed, and an ulcer is formed. Such an ulcer has an elevated border, sharply cut edges, an indurated base, and exudes a scanty serous discharge; its surface is at first occupied by yellow necrosed tissue, but in time this is replaced by smooth, pale-pink granulation tissue; finally, epithelium may spread over the surface, and the ulcer heals. As a rule, the patient suffers little discomfort, and may even be ignorant of the existence of the lesion, unless, as a result of exposure to mechanical or septic irritation, ulceration ensues, and the sore becomes painful and tender, and yields a purulent discharge. The primary lesion may persist until the secondary manifestations make their appearance, that is, for several weeks. It cannot be emphasised too strongly that the induration of the primary lesion, which has obtained for it the name of "hard chancre," is its most important characteristic. It is best appreciated when the sore is grasped from side to side between the finger and thumb. The sensation on grasping it has been aptly compared to that imparted by a nodule of cartilage, or by a button felt through a layer of cloth. The evidence obtained by touch is more valuable than that obtained by inspection, a fact which is made use of in the recognition of _concealed chancres_--that is, those which are hidden by a tight prepuce. The induration is due not only to the dense packing of the connective-tissue spaces with lymphocytes and plasma cells, but also to the formation of new connective-tissue elements. It is most marked in chancres situated in the furrow between the glans and the prepuce.