ON THE RELATIONS OF MICRO-ORGANISMS TO DISEASE 301 
the organism has been cultivated, and a drop of the liquid has been placed under 
a microscope on the table. It will be seen that the organism is a minute form 
of bacterium, oval-shaped, tending to multiplication by transverse constriction, 
and very frequently seen in pairs, and occasionally in chains. Its transverse 
diameter is from I-50,oooth to I-25,oooth of an inch, so that it resembles very 
closely the Bactertum lactis. The woodcut d represents a camera-lucida sketch 
of the organism sent by M. Pasteur. It is drawn on the same scale as the other 
illustrations of this paper. So far as I am aware, this is the first time this 
bacterium has been shown in this country. Now, it was found by Pasteur 
that the organism could be produced in chicken broth in any number of suc- 
cessive cultivations, and to the last retained its full virulence, so that, if a healthy 
chicken was inoculated with it, the fatal disease was produced as surely as by 
inoculation with the blood of a fowl that had died of the complaint. This was 
pretty conclusive evidence that the organism was the cause of the disease, and 
that it constituted the true infective element ; because any other material that 
might be supposed to accompany it in the blood of the diseased animal must 
have been got rid of by the successive cultivations in chicken broth. 
The growth of the organism occasions no putrefaction in the liquid; so 
that this is a good example of a bacterium which is most destructive as a disease, 
but which is at the same time entirely destitute of septic property, in the primi- 
tive sense of that term as equivalent to putrefactive. After the bacterium 
has grown for a certain time in a given portion of chicken broth, it ceases to 
develop further ; and when this is the case, although the broth has lost only 
a very small proportion of its substance by weight, and although, as aforesaid, 
it has not undergone putrefaction, and still constitutes an excellent pabulum 
for ordinary forms of bacteria, the bacterium of the fowl-cholera, though intro- 
duced from some new source, is incapable of growing in it. This fact seems 
highly suggestive of an analogy with the effects of vaccination, or those of an 
attack of measles or scarlatina, in securing immunity from the disease for the 
future. Here we have a certain medium invaded by a virus capable of self- 
multiplication, as is the case with those diseases in the animal body ; the medium 
itself little affected chemically by the growth of the virus within it, but never- 
theless rendered unfit for the development of that virus for the future. But 
something more than the suggestion of analogy with vaccination has been 
