[Illustration: FIG. 41.—Dark-field view of _Leptospira icteroides_ in a culture 16 days old. × 1000. (After Noguchi.)] Autopsies of the guinea pigs showed a fatty and yellow liver with nephritis. Some of the injected guinea pigs showed only fever but seemed to have acquired an immunity to subsequent injections of a virulent virus. In the blood, liver and kidneys of the animals showing the jaundice and albuminuria Noguchi was able to demonstrate spirochaetes by dark-field illumination. He also obtained cultures from such animals. Leptospiras were demonstrated in the blood of 3 out of 27 human cases but only after prolonged search. Cultures have been obtained from human yellow fever blood. For culturing, a medium is used consisting of 1 part of serum and 3 parts of Ringer’s solution made semisolid with 0.3% agar and contained in tall tubes. One cc. citrated yellow fever blood is introduced into the lower part of the medium. A thin layer of liquid petrolatum is poured on the top of the medium. We need partial oxygen tension but not anaerobiasis. Optimum growth temperature is 33°C. _L. icteroides_ is from 4 to 9 microns long by 0.2 wide and tapers gradually to extremely thin sharp points. These organisms will pass the pores of V and N Berkefeld filters thus placing them in the group of filterable viruses. The virulence of different strains varies, with some strains as little as 0.00001 cc. of culture proving fatal for guinea pigs. Monkeys, rabbits and birds were not susceptible to infection but the marmoset and puppy seemed to respond as did the guinea pig. By having _Stegomyia_ (_Aedes calopus_) feed on infected guinea pigs as well as the human case of yellow fever, and subsequently allowing these mosquitoes to bite normal guinea pigs, the disease was transmitted in a few cases. In certain infected mosquitoes Noguchi found leptospiral organisms with dark-field illumination. Mosquitoes fed on infected guinea pigs became infectious in 8 days, this shorter period over human feedings being due, probably, to the greater abundance of organisms in the blood of the infected guinea pigs over human yellow fever blood. Guiteras doubts the etiological relation of Noguchi’s leptospira to yellow fever on the ground that in view of the susceptibility of animals to this organism the disease should exist as an epizootic, which is improbable in view of the ease with which yellow fever is eradicated when measures applied solely to man and the mosquito are practiced. Among other points of objection he notes the recovery of the spirochaete from the blood later than the third day of the disease. [Illustration: FIG. 42.—_Leptospira icteroides_ in blood of a guinea pig inoculated with a culture × 1000. (After Noguchi.)]