SUSCEPTIBILITY AND IMMUNITY. 247 
troduced upon the surface of the brain, and immunity is established 
during the interval—so-called period of incubation—which usually 
occurs between the date of infection and the development of the 
disease. That the immunity in this case also depends upon the 
introduction of a chemical substance present in the desiccated spinal 
eord of rabbits which have succumbed to rabies, which is used in 
these inoculations, is extremely probable. But, as the germ of rabies 
has not been isolated or cultivated artificially, this has not yet been 
demonstrated. Wooldridge claims to have made susceptible animals 
immune against anthrax by inoculating them with an aqueous ex- 
tract of the testicle or of the thymus gland of healthy animals. 
We may mention also the interesting results obtained by Em- 
merich, Freudenreich, and others, who have shown that an anthrax 
infection in a susceptible animal inoculated with a virulent culture 
may be made to take a modified and non-fatal course by the simul- 
taneous or subsequent inoculation of certain other non-pathogenic 
bacteria—streptococcus of erysipelas, Bacillus pyocyaneus. 
In a series of experiments made by the writer some years ago 
evidence was obtained that, under certain circumstances, immunity 
from the effects of one pathogenic bacillus may be obtained by the 
previous injection of a pure culture of a different species. In the 
experiments referred to injections into the cavity of the abdomen of 
a culture of Bacillus pyocyaneus protected rabbits from the lethal 
effects of Bacillus cuniculicida Havaniensis, when subsequently in- 
jected into the cavity of the abdomen in such amount (one cubic 
centimetre of a bouillon culture) as invariably proved fatal in rabbits 
not protected by such injections. 
Before considering the theories which have been offered in expla- 
nation of acquired immunity it is desirable to call attention to certain 
observations which have been made during the past few years relat- 
ing to “‘ chemiotaxis.” 
The term chemotaxis was first used by Pfeffer to designate the 
property, observed by himself and others, which certain living cells 
exhibit with reference to non-living organic material, and by virtue 
of which they approach or recede from certain substances. The 
chemiotaxis is said to- be positive when the living cell approaches, and 
negative when it recedes from, a chemical substance. As examples 
of this we may mention the approach of motile bacteria to nutrient 
material or to the surface of a liquid medium where they find the 
oxygen required for their vital activities ; and of leucocytes to cer- 
tain substances when these are introduced beneath the skin of warm- 
or cold-blooded animals. This subject has recently received much 
