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My theory of the transmission of yellow fever will now be explained. 

 The female mosquito having introduced its lance into the skin of a 

 yellow fever patient through one of its pores (excretory ducts of the 

 sebacious or sudoriparous glands) pierces one of the blood capillaries 

 and fills with blood. In so doing the transverse' ridges and the terminal 

 teeth that exist on the outside of the compound lance of the insect are 

 supposed to pick up one of the disease germs contained either in the 

 blood itself, in the walls of the capillaries, in the connective tissue, or in 

 the excretory duct of the cutaneous glands through which it has penetrated. 

 After this operation the lanee is withdrawn within its sheath, and the 

 mosquito, weighted by the blood, seeks some dark corner where it may 

 hide and digest unmolested the blood that it has sucked. Forty-eight hours 

 in summer, and three to five days in winter are required for this digestion, 

 during which time the insect persistently refuses to sting again, though it 

 is always ready to suck, with the point of its proboscis, at any particles of 

 sugar that may be within its reach. The female mosquito has generally 

 been fecundated before it begins to sting, and will therefore seek some pool 

 of stagnant water or neglected bucket in the open air, where it may lay 

 its eggs. The day mosquito never lays its eggs in the manner described in 

 books, but scatters them, besmeared with a glutinous substance, over the 

 surface of the water or upon the sides of the tank or vessel. Its colour is 

 black or steel; there are five white rings on its hind legs, and others on 

 the middle and front ones; its wings are so short that they do not cover 

 the anal segment of its body. In all these particulars it differs from the 

 night mosquito, which is larger in size, of a uniform yellowish-brown colour, 

 lays its eggs like the European Culex pipiens, and has longer wings. I have 

 only experimented with the diurnal and crepuscular species, for the 

 nocturnal can seldom be made to sting more than once. 



The C. mosquito may be kept alive during thirty-five days in a very 

 small space, and with a very scanty supply of air, provided it is allowed 

 to fill itself with blood every two or three days, or if only some dry sugar 

 and a supply of fresh water are placed within its reach. 



In my inoculation experiments the interval between the application 

 of the contaminated mosquito to a susceptible person and the appearance 

 of the first symptoms of a mild attack of the disease ( when such a one did 

 occur) has varied between five and twenty-five days, the latter term 

 being the one that I have fixed upon, beyond which any morbid symptoms 

 would be considered as independent of the inoculation. In the majority of 

 cases no pathogenic effects were produced — only one or two recently con- 

 taminated mosquitoes having been used in each case — but a protective in- 

 fluence appears to have been thereby manifested, as will be shown m the 

 next section, where artificial immunity will be considered. 



Natural and Artificial Immunity. — Some individuals appear to be 

 entirely refractory to the yellow fever infection, and among those who are 



