60 THE CIRCULATING LIQUIDS OF THE BODY 



absorption of tissue by the aid of leucocytes is the removal of the 

 necrosed decidua reflexa, the fold of uterine mucous membrane 

 which envelops the ovum (Minot). 



The behaviour of phagocytes towards pathogenic micro-organisms 

 is of even greater interest and importance. Metchnikoff laid the 

 foundation of our knowledge of this subject by his researches on 

 Daphnia, a small crustacean with transparent tissues, which can be 

 observed under the microscope. When this creature is fed with a 

 fungus, Monospora, the spores of the latter find their way into the 

 body-cavity. Here they are at once attacked by the leucocytes, 

 ingested, and destroyed. But after a time so many spores get 

 through that the leucocytes are unable to deal with them all ; some 

 of them develop into the first or ' conidium ' stage of the fungus ; the 

 conidia poison the leucocytes, instead of being destroyed by them, 

 and the animal generally dies. Occasionally, however, the leuco- 

 cytes are able to destroy all the spores, and the life of the Daphnia is 

 preserved. This battle, ending sometimes in victory, sometimes in 

 defeat, is believed by Metchnikoff to be typical of the struggle which 

 the phagocytes of higher animals and of man seem to engage in 

 when the germs of disease are introduced into the organism. He 

 supposes that the immunity to certain diseases possessed naturally 

 by some animals, and which may be conferred on others by vaccina- 

 tion with various protective substances, is, to a large extent, due to 

 the early and complete success of the phagocytes in the fight with 

 the bacteria; and that in rapidly- fatal diseases such as chicken- 

 cholera in birds and rabbits, and anthrax in mice the absence of 

 any effective phagocytosis is the factor which determines the result. 

 Others have laid stress on the action of protective substances sup- 

 posed to exist in the plasma itself. It is possible that such sub- 

 stances are manufactured by the leucocytes, and either given off by 

 them to the plasma by a process of ' excretion/ or liberated by their 

 complete solution. 



The most recent investigations go to show that Metchnikoff's 

 phagocytic theory of immunity requires modification, at any rate in 

 the case of the higher animals and man, although the brilliant 

 biological observations on which it was originally built retain all 

 their value. He supposed that in the immunizing process the 

 leucocytes underwent certain changes, acquired, so to speak, a sort 

 of ' education ' that enabled them to cope with bacteria against 

 which they were previously powerless. It seems more probable 

 that in the presence of the substances that confer immunity, not only 

 the leucocytes, but other cells, are stimulated to produce bodies 

 which cut short the life, or inhibit the growth, of the bacteria 

 (alexins), or prepare them for being taken up by the phagocytes 

 (opsonins). It has been shown that bacteria which have been in 

 contact with serum containing the appropriate opsonins are taken 



