364 SOME POINTS OF GENEEAL INTEEEST 
which are diseases due to microbes which invade the blood 
stream acutely, and are found in large numbers over the whole 
body. The plague bacillus, as seen under the microscope in 
plague lesions, shows a considerable polymorphism. Three 
forms predominate : short oval rods, long rods, and large 
oval, pear-shaped or round involution or degenerate forms, 
which only take on stains faintly. In pure cultures all these 
forms are also met with. In staining the B. pestis, we find 
a peculiarity which separates it from most other bacilli, though 
not from all, and that is the phenomenon of what is known 
as ‘ polar staining.’ The micro-organism appears stained at 
the ends, but not at all, or only slightly so, in the middle. 
This, for diagnostic purposes, is a striking and important 
point. 
Now let us see how the bacteriologist would proceed to 
grow this bacillus. He would employ several methods. First 
he would take a little material from, say, the bubo, and streak 
it on a gelatinous substance made from seaweed, known as 
agar. If he added 2 to 3 per cent, of salt to this agar, 
he would find that the characteristic involution forms I have 
just described would be readily obtained ; this feature dis- 
tinguishing it from other bacilli. He would next proceed 
to grow the bacillus in broth, especially in broth on which 
oil-drops were floating. After a time, he would find a growth 
as a powdery thread, in regular stalactite forms, hanging 
from the under surfaces of the oil-drops. This, again, to his 
mind, would form a characteristic and diagnostic feature. 
(I shall remind you later on of these stalactite forms when 
I am speaking on plague vaccines.) 
You will wonder why the bacteriologist takes all this 
trouble. Are there, then, other bacilli so like plague in their 
forms, and in the way they grow on these cultures, as to be 
mistaken for it ? There are several. For instance, the bacilli 
that causes swine plague ; but in particular, one with a name 
far longer than itself, the Bacillus pseudotuberculosis rodentium, 
which is moderately pathogenic to rats, but not to human 
beings. This so closely resembles B. pestis, not only in micro- 
scopical but in cultural tests, that we have to resort to the 
cutaneous inoculation of white rats to differentiate it. We 
