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.