ed largely
on conjecture, the one brilliant exception being the discovery of
vaccination by Jenner in 1798.

The host possesses the passive defences of the surfaces which have
already been considered. The first theories advanced in explanation of
immunity were influenced by what was known of fermentation. One, the
exhaustion theory, assumed that in the course of disease substances
contained in the body and necessary for the growth of the bacteria
became exhausted and the bacteria died in consequence. Another, the
theory of addition, assumed that in the course of the disease
substances inimical to the bacteria were formed. Both these theories
were inadequate and not in accord with what was known of the
physiology of the body. The most general mode of defence is by
phagocytosis, the property which many cells have of devouring and
digesting solid substances (Fig. 16-p). Although this had been known
to take place in the amoebae and other unicellular organisms, the wide
extent of the process and its importance in immunity was first
recognized by Metschnikoff in 1884 and the phagocytic theory of
immunity advanced and defended by a brilliant series of experiments by
Metschnikoff and his pupils conducted in the Pasteur Institute.
Metschnikoff's first observations were made on the daphnea, a small
animalcule just visible to the naked eye which lives in fresh water.
The structure of the organism is simple, consisting of an external and
internal surface between which there is a space, the body cavity;
daphneae are transparent and can be studied under the microscope while
living. Metschnikoff observed that certain of them in the aquarium
gradually lost their transparency and died, and examining these he
found they were attacked by a species of fungus having long, thin
spores. These spores were taken into the intestine with other food;
they penetrated the thin wall of the intestine, passed into the body
cavity, multiplied there, and in consequence the animal died. In many
cases, however, those penetrating became enclosed in cells which the
body cavity contains and which correspond with the leucocytes of the
blood; in these the spores were digested and destroyed. The daphneae in
which this took place recovered from the infection. Here was a case in
which all the stages of an infectious disease could be directly
followed under the microscope, and the whole process was simple in
comparison with infections in the higher animals. The pa