THE CELLULAR MECHANISMS OF DEFENCE 1071 



even from salts of ammonia. It is probable that in these cases the animal is 

 decidedly benefited from the presence of these bacteria in its alimentary 

 canal, so that here also we may speak of a symbiosis. In most cases invasion 

 of a higher animal or plant by some lower organism is fraught with danger to 

 the host, so that special mechanisms have to be provided for the protection 

 of the tissues from infection. The most primitive means of defence, and 

 one which is found throughout the whole animal kingdom, is exactly ana- 

 logous to the process by which the amoeba destroys and utilises any bacteria 

 present in its environment. The prevention of infection is of course the 

 function of the external layers of the organism, i.e. the epithelial covering, 

 either of the skin or of the surface of the gut. Protection here may be 

 of a physical or chemical character. The cells may secrete a horny or 

 chitinous layer which presents a mechanical obstruction to the entry of 



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FIG. 491. A, amoeba, infected by Microsphcera : a, early stage. B, a dying 

 amoeba, full of parasitic Microsphceroz. (METCHNIKOFF. ) 



bacteria. They may secrete mucin, which entangles and hinders the move- 

 ments of invading micro-organisms, or they may secrete substances which 

 actually destroy the life of such organisms. When however a micro- 

 organism has obtained entrance to the interior of the body, e. g. through a 

 wound of the surface epithelium, the task of dealing with the invader 

 becomes the office of a special type of cells belonging to the mesoblast. 

 These cells are similar in character to the amoeba. They have the power 

 of extruding pseudopodia, of wandering from place to place, and of englobing 

 and digesting particles of food or bacteria with which they come in contact. 

 On account of these latter properties they have been called by Metchnikofi 

 phagocytes, and the whole process by which foreign material or the animal's 

 own dead tissues are got rid of is spoken of as phagocytosis. The process can 

 be well studied, as has been shown by Metchnikoff, in the sponge or in the 

 larva of the echinoderm. At one stage in the development of the latter the 

 larva consists of a sac which is involuted at one extremity to form the 



