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ease of Yellow Fever was running its course; if present, the disease would 

 be transmissible, if absent the disease would not extend to other susceptible 

 persons around the patient. The first step, under that hypothesis, was to 

 discover such an agent, bearing in mind that besides fulfilling the above 

 conditions it must likewise satisfy all those that are known to influence the 

 propagation of the disease. 



It must be susceptible of establishing only a temporary existence in 

 places where the disease only occurs as a casual epidemic, and its existence 

 or functional activity must be incompatible with the climatic or topogra- 

 phic conditions of certain localities where yellow fever is never 

 transmissible. 



1 have not been able to think of any other agent that might comply 

 with the requisites except the tropical mosquito of America, and I must 

 say that so Ear it has corresponded to the various tests by which I have 

 tried it in a manner far exceeding my first expectations. 



I must premise by stating that my investigations have been limited to 

 the diurnal ami crepuscular species, the Culex Mosquito of Havana (so 

 named by Robinau Desvoidy, in 1817 or 1820. from specimens carried to 

 Paris by the Cuban naturalist Felipe Poey), which 1 need not describe 

 here having done so in my first communication to this Journal. For some 

 reason that I cannot explain, that species is the only one from which I have 

 obtained a succession of stings (as many as twelve or more) in the 30 or 

 40 days that 1 have kept them alive in small glass phials. 



Put for Dr. Sternberg's insistanee on that point it would be needless 

 to say that it is not from the blood that has been sucked that the mosquito 

 is supposed to derive its contamination, but from the tissues that the sting 

 must bore through, from the perivascular lymph or may be from the 

 contents of the excretory ducts of the sudoriparous or sebaceous glands 

 through which the insect may find a readier entrance for the introduction 

 of its stings. It is therefore on the outer surface of the latter, upon the 

 terminal teeth on the transverse serrated grooves of the mandibles, that we 

 should expect the germs to be retained. The two mandibles together with 

 the two maxillae, the hollow labrum and the hypopharynx combine to form 

 the wiry sting that lies enclosed and hidden From our view within the 

 dark, thick sheath which is all that is seen of the proboscis, except at 

 such times as when the stint;- has to be protuded in order to perform its 

 burrowing functions. The antiseptic besmearing of the proboscis could not 

 therefore interfere with the development of germs situated in the interior 

 of the sheath, apon the surface of the sting itself. 



By the term "contaminated mosquito" I understand such as have stung 

 a yellow fever patient during the first six days of the disease, and it is my 



belief that whereas on • two stings from mosquitoes recently 



contaminated may either occasion in susceptible persons a mild attack or 

 simply confer immunity without any pathogenic manifestations, a severe 



