872 SOME POINTS OP GENERAL INTEREST 
body disintegrate, allowing the harder shell, which is composed 
of chitin, to be separated into segments. Microscopical speci- 
mens can be prepared by boiling the fleas in glycerine 
and mounting them in that medium, or they may be 
boiled in alcohol, cleared in cedar -wood oil, and mounted 
in Canada balsam. 
The Process by which the Flea transfers Infection. — Now 
let us introduce the Bacillus pestis into the flea — or rather he 
will do that himself, given the chance. The average capacity 
of a rat flea’s stomach is about 0-5 c.mm., and on this basis 
a flea may take in as many as 5000 plague germs into his 
stomach after imbibing the blood of a plague rat. Moreover, 
multiplication of plague bacilli takes place in the stomach of 
the rat flea. This multiplication varies with the season of 
the year. In the epidemic season it is six times greater than 
in the non-epidemic. Plague bacilli are present in the rectum 
and faeces of fleas taken from plague rats, and such faeces are 
infective to guinea-pigs, both by the cutaneous and sub- 
cutaneous methods of inoculation. Plague bacilli have been 
found in the oesophagus of the flea, but never in any other 
region of the body, such as the body cavity or salivary glands. 
A single flea may transmit the disease, and both male and female 
fleas can do so. 
Two modes of transmission have been proved. (1) The 
flea defaecates while he is yet sucking blood, and the rubbing 
of this material into the puncture brings about infection. 
(2) Regurgitation, as the result of masses of plague bacilli in 
the oesophagus of the flea, causes injection of plague bacilli 
into the rat or man in the act of biting, and this is the more 
important mode. 
Experiments showing that the Flea is the Transmitting Agent 
of Infection. — 1. Certain experiments which w T ere carried out in 
godowns in Bombay went to prove that close and continuous 
contact of plague-infected animals with healthy animals, if 
fleas are excluded, does not give rise to an epizootic among 
the latter. As the godowns were never cleaned out during 
the experiments, close contact included contact with faeces 
and urine, and the eating of food contaminated with these 
materials. 
