SUSCEPTIBILITY AND IMMUNITY. 251 
tation by their accumulation in the fermenting liquid, such as alco- 
‘ hol, lactic acid, phenol, etc., would not be so retained. But we can- 
not speak so positively with reference to the toxic albuminous 
substances which recent researches have demonstrated to be present 
in cultures of some of the best known pathogenic bacteria. It is 
difficult, however, to believe that an individual who has passed 
through attacks of half a dozen different infectious diseases carries 
about with him a store of as many different chemical substances pro- 
duced during these attacks, and sufficient in quantity to prevent the 
development of the several germs of these diseases. Nor does the 
experimental evidence relating to the action of germicide and germ- 
restraining agents justify the view that a substance capable of 
preventing the development of one microérganism should be with- 
out effect upon others of the same class; but if we accept the re- 
tention hypothesis we must admit that the inhibiting substance 
produced by each particular pathogenic germ is effective only in 
restraining the development of the microbe which produced it in the 
first instance. 
Pasteur discusses this hypothesis in his paper from which we 
have already quoted, as follows : 
‘“We may admit the possibility that the development of the microbe, in 
place of removing or destroying certain mattersin the bodies of the fowls, 
adds, on the contrary, something which is an obstacle to the future develop- 
ment of this microbe. The history of the life of inferior beings authorizes 
such a supposition. The excretions resulting from vital processes may arrest 
vital processes of the same nature. In certain fermentations we see anti- 
septic products make their appearance during, and as a result of, the fer- 
mentation, which put an end to the active life of the ferments and arrest 
the fermentations long before they are completed. In the cultivation of our 
microbe, products may have been formed the presence of which, possibly, 
may explain the protection following inoculation. 
“Our artificial cultures permit us to test the truth of this hypothesis. 
Let us prepare an artificial culture of the microbe, and after having evapo- 
rated it, in vacuo, without heat, let us bring it back to its original volume 
by means of fresh chicken bouillon. If the extract contains a poison for 
the life of the microbe, and if this is the cause of its failure to multiply in the 
filtered liquid, the new liquid should remain sterile. Now, thisis not the case. 
We cannot, then, believe that during the life of the parasite certain substances 
are.produced which are capable of arresting its ulterior development.” 
This experiment of Pasteur appears to be conclusive so far as the 
particular pathogenic microérganism referred to is concerned ; and 
we may say, in brief, that more recent inves‘igations do not sustain 
the view that acquired immunity is due to the retention of products 
such as are formed by pathogenic bacteria in artificial culture media, 
and which act by destroying these bacteria or restraining their devel- 
opment when they are introduced into the bodies of immune animals. 
Moreover, if we suppose that the toxic substances which give 
pathogenic power to a particular microérganism are retained in the 
