Castellani and others have shown that monkeys successfully inoculated with syphilis do not become immune to yaws, and vice-versâ. Craig and Ashburn, using the monkey _Cynomolgus philippinensis_, found these animals susceptible to yaws but not to syphilis. The ulcerated lesions of frambœsia are rapidly invaded by numerous bacteria as well as by different spirochætes, of which Castellani has described three distinct species. One is identical with _Spirochæta refringens_, Schaudinn, the other two are thin and delicate. One, _S. obtusa_, has blunt ends; the other _S. acuminata_, has pointed ends. _T. pertenue_ is also present. The reasons for considering _T. pertenue_ to be the specific cause of frambœsia are:-- (1) _T. pertenue_ is the only organism present in non-ulcerated papules, in the spleen and in the lymphatics of yaws patients, or of monkeys artificially infected with the disease. By no method has any other organism been obtained. (2) Extract of frambœsia material, free from all organisms other than _T. pertenue_, reproduces the disease if inoculated. (3) Extract of frambœsia material deprived by filtration of _T. pertenue_ is no longer infective on inoculation. The method of infection is contaminative, by direct contact. Women in Ceylon are frequently infected by their children. Any slight skin abrasion is sufficient to admit the parasite. In some cases, insects may carry the disease from person to person, and even in hospitals, when dressings are removed, it has been noticed that flies greedily suck the secretion from the ulcers. _T. pertenue_ has been recovered from flies that have fed on yaws, and monkeys have contracted the disease when flies were placed and retained on them for a short time, after the insects had fed on yaws material. CULTIVATION.--_T. pertenue_ has been cultivated by Noguchi, who finds three types of parasites in his cultures, as before mentioned. Its multiplication is by longitudinal division. Noguchi[169] (1912), has cultivated species of Treponema from the human mouth, e.g., _T. macrodentium_, _T. microdentium_ and _T. mucosum_, the latter from pyorrhea alveolaris. These parasites in the past may have been confused under the name _Spirochæta dentium_. [169] _Journ. Exptl. Med._, xv, p. 81; xvi, p. 194. Class III. *SPOROZOA*, Leuckart, 1879. The third group of the Protozoa consists entirely of parasitic organisms forming the class known as the Sporozoa or spore-producing animals. The members of this class are characterized by possessing very great powers of multiplication, coupled with a capacity for producing forms that serve for the transference of the organisms to other hosts. These reproductive bodies, whether for increase of numbers within one host or for transmission to another host, are called spores. But, strictly, the term spore should be used only in the latter connection, when a protective or resistant coat known as a sporocyst envelops the body of the spore.