_Treponema pallidum_ was first described by Schaudinn and Hoffmann in 1905 under the name of _Spirochæta pallida_. It has also been described under the names of _Spironema pallida_, _Microspironema pallida_ and _Trypanosoma luis_. Siegel in 1905 described an organism which he called _Cytorhyctes luis_ and considered to be the agent of syphilis. Schaudinn reinvestigated Siegel’s work and found _T. pallidum_, which he considered to be the causal agent of the disease, and pronounced against _Cytorhyctes luis_. It is probable now that both workers were correct, for Balfour (1911) has seen the emission of minute granules or “coccoid” bodies from _T. pallidum_ and these granules probably correspond to the _C. luis_ of Siegel. Recently E. H. Ross, having observed a spirochæte stage in the development of Kurloff bodies, thinks that _T. pallidum_ is a stage in the life-history of a Lymphocytozoon. MacDonagh has also described a complicated and somewhat similar cycle, but these observations require further study and confirmation. [Illustration: FIG. 56.--_Treponema pallidum_. (After Bell, from Castellani and Chalmers.)] _T. pallidum_ varies from 4 µ to 10 µ in length, its average length being 7 µ, while its width is usually about 0·25 µ. Longer individuals of 16 µ to 20 µ have been recorded. The body has from eight to ten spiral turns and forms a tapering process at each end (fig. 56). The organism is most difficult to stain, and its internal structure is little known. It is possibly like that of _Spirochæta duttoni_ or _S. balbianii_, as the “granule shedding” observed by Balfour is strongly suggestive of the formation of resistant bodies by those spirochætes. Hoffmann (1912) has seen the formation of spores in _T. pallidum_. The Treponemata occur in the primary and secondary sores, but are difficult to find in the tertiary eruptions of syphilis. Noguchi and Moore (1913) and Mott[164] (1913) have demonstrated _T. pallidum_ in the brain in cases of general paralysis of the insane. Marie and Levaditi (1914), however, consider that the treponeme found in the brain in such cases is different from _T. pallidum_. [164] _Brit. Med. Journ._, Nov. 15, 1913. p. 1, 271. CULTIVATION _of T. pallidum_.--This has been accomplished successfully by Noguchi,[165] using a modification of his method for spirochæte cultivation, for _T. pallidum_ is much more difficult to grow than spirochætes, being a strict anaerobe. [165] _Journ. Exptl. Med._, xv, p. 90; xvi, p. 211. [Illustration: FIG. 57.--Diagram of apparatus for cultivation of _Treponema pallidum_ by Noguchi’s method. (After Noguchi.)] The apparatus consists of two glass tubes, the upper being connected to the lower by a narrower tube passing through a rubber cork (fig. 57). Both tubes are carefully sterilized.