52 A MANUAL OF PHYSIOLOGY 



structures which in the course of development have become 

 obsolete, or with the neutralization or elimination of harmful 

 substances introduced from without, or formed by the activity 

 of bacteria within the tissues. During the metamorphosis of 

 some larvae, groups of cilia and muscle-fibres may be absorbed 

 and eaten up by the leucocytes. In the metamorphosis of 

 maggots, for example, the muscular fibres of the abdominal 

 wall, which are used in creeping, and are therefore not required 

 in the adult, degenerate, and are devoured by swarms of leuco- 

 cytes which migrate into them. In the human subject an 

 example of 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. Metschni- 

 koff 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 leucocytes 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 Metschnikoff 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 vaccination 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 supposed to exist 

 in the living plasma itself, although only as yet demonstrated in 

 the serum. It is possible that such substances 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 Metschnikoff 's 

 phagocytic theory of immunity requires modification, at any 



