THE CIRCULATING LIQUIDS OF THE BODY 55 



called phagocytes ; and it is to be remarked that this term 

 neither comprises all leucocytes nor excludes all other cells, 

 for some fixed cells, such as those of the endothelial lining 

 of bloodvessels, are phagocytes in virtue of their power of 

 sending out protoplasmic processes, while the small, im- 

 mobile, uninuclear leucocyte, or lymphocyte, is not a phago- 

 cyte. 



Although it is not at present possible to assign a physio- 

 logical value to all the phenomena of phagocytosis, either as 

 regards the phagocytes themselves or as regards the 

 organism of which they form a part, there seems little doubt 

 that under certain circumstances the process is connected 

 with the removal of structures which in the course of 

 development have become obsolete, or with the neutraliza- 

 tion 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 leucocytes which 

 migrate into them. 



But the behaviour of phagocytes towards pathogenic 

 micro-organisms is of even greater interest and importance. 

 Metschnikoff 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 leuco- 

 cytes, 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 

 conidiurn 3 stage of the fungus ; the conidia poison the 

 ieucGcytes, inscead of being destroyed by them, and the 

 animai generally dies. Occasionally, however, the leuco- 

 cytes ar? able to destroy all the spores, and che life of the 

 Daphnia is preserved. This battle, ending sometimes in 

 victory, sometimes in defeat, is believed by Metschnikoff to 



