184 MR. RICHARD S. BAGNALL ON THE SIPHONAPTERA 



of the flea. All attempts have signally failed, however, to 

 establish such a connection. 



(2) It also appeared that, except just after a flea had been 

 feeding on a plague-stricken animal, no bacilli could be 

 detected in the alimentary tract between the mouth and the 

 gizzard. The bacilli were numerous, so numerous as to 

 prove that multiplication had taken place, within the stomach, 

 but the valve formed by the gizzard is so tight that no one 

 believed in the possibility of bacilli from the stomach entering 

 the host %'ia the mouth parts of the flea. 



(3) It was noticed that during a meal the contents of the 

 gut were discharged from time to time. These foecal drops 

 contain numerous bacilli, and experiment showed that 

 infection might be brought about either by the movements of 

 the flea itself, on the completion of its meal, or by the host's 

 rubbing at the angry spot. This first method of infection 

 received a very qualified support, even by its discoverers, 

 because in spite of the evidence from dissection, infection by 

 bite alone appeared to be taking place. 



(4) Recently (1914) a new light has been shed on the 

 matter by the discoveries of Messrs. Bacot and Martin, of the 

 Lister Institute. In experimenting with X, cheopis and 

 C. fasciatus a certain number of the insects were found to be 

 suffering from obstruction of the alimentary canal. Further 

 investigation showed that, in such cases, the plague bacilli had 

 multiplied so excessively in the gizzard and stomach as to 

 form an impassable jelly-like mass. Fleas aff'ected in this 

 way were thirstier and more reckless than normal individuals. 

 When placed on a host they sucked vigorously for a time, 

 but tlie blood taken up failed to pass into the stomach. Then, 

 wlien the sucking eff'ort ceased, the blood which had been 

 imbibed, now contaminated with plague bacilli, was forced 

 back into the wound, and infection took place. The first 

 two methods — the first comparable to vaccination, the second to 

 inoculation -are amply sufficient to explain how fleas convey 



