_B. icteroides_ grows as readily on culture media as does the typhoid organism. It was also shown that yellow fever blood, which was negative for _B. icteroides_, could produce yellow fever when injected subcutaneously. In 11 autopsies made shortly after death cultures from various viscera were negative for _B. icteroides_. It was then decided to abandon, as fruitless, further investigations as to Sanarelli’s organism, and to take up the mosquito transmission theory. =Mosquito Transmission.=—In 1848, Doctor Nott, of Mobile, strongly advocated the insect transmission of yellow fever but not specifically incriminating the mosquito. Riley from a study of Nott’s original paper, thinks that the author had in mind invisible forms of insect life, which could act as disease producers, and simply cited the mosquito to illustrate his views. In 1853, Dr. Beauperthuis, in Guadeloupe, noted that malaria and yellow fever ceased to exist in regions which from their altitude fail to nourish “insectes tipulaires.” He also thought that the virus of these diseases was introduced by the channel of insect inoculation. Furthermore he stated that the variety, zancudo bobo (_Stegomyia_), had white stripes on the legs and was in a way the domestic species. However, there is a question whether his zancudo bobo was _Stegomyia_ and furthermore if we translate the expression _inoffensif_ as without result it would negate the connection between this mosquito and disease production. From 1881 Doctor Carlos Finlay had been advocating the transmission of yellow fever by _Culex fasciatus_ (Synonym for _Stegomyia calopus_). In 100 experiments made by Finlay 13 cases of yellow fever developed. In no instance had these experimental mosquitoes fed on a yellow fever patient more than six days previous to their biting and, knowing that a period of at least twelve days must elapse, the infections in the 13 cases could not have been brought about by these experimental mosquitoes. =Extrinsic Incubation.=—An observation by Carter influenced the Commission in their investigations. Carter, in 1898, noted that a period of about two weeks generally elapsed between the appearance of the first case and the group of cases resulting from this first case. He termed this period the “extrinsic incubation” and we now know this as synonymous with the twelve-day period of incubation in the mosquitoes infected by the first case plus the two to five days of the period of incubation in man.