1901] MICROSCOPICAL JOURNAL. 39 
animals, took up large numbers of the plague bacilli and 
retained them foranumber of days. These bugs did not, 
however, infect healthy animals when allowed, subse- 
quently, to feed upon them. Nuttall is not, however, sat- 
isfied that the number of his experiments upon this point 
was great enough to be conclusive. Ogata found that the 
plague bacillus existed in the bodies of fleas found upon 
diseased rats. One of these he crushed between sterile 
object-glasses and introduced into the subcutaneous tis- 
sues of a mouse, which died in three days with typical 
lesions of the plague, a control animal remaining well. 
The guinea-pigs taken for experimental purposes into a 
plague district, and kept carefully isolated, died sponta- 
neously of the disease, presumably because of insect in- 
fection. The animal most prone to spontaneous infection 
seems to be the rat, and there is much evidence in sup- 
port of the view, that it aids in the spread of epidemics. 
At several of the Asiatic plague districts and at Santos 
the appearance of plague among the inhabitants was pre- 
ceded by a large mortality among the rats, some of which 
when examined, showed bubces and had died of plague- 
septicemia. It is rather improbable that men become in- 
fected with plague through the bites of the fleas, leaving 
- the bodies of plague-destroyed rats, as was once supposed. 
Galli-Valerio thinks the fleas of the mouse and rat are in- 
capable of living upon man and do not bite him, and that 
it is only the Pulew irritans, or human flea, that is ca- 
pable of transmitting the disease from man to man. Yer- 
sin found that when cultivated for any length of time 
upon culture media, especially agar-agar, the virulence 
was rapidly lost and the bacillus eventually died. On the 
other hand, when constantly inoculated from animal to 
animal, the virulence of the bacillus is much increased. 
The bacillus probably attenuates readily. Kitasato says 
that it did not seem able to withstand dessication, longer 
than four days ; but Rappaport (quoted by Wyman) found 
