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THE BRITISH NATURALIST. [September 



clearly shown that the introduction of tubercle bacilli into a 

 guinea-pig is rapidly followed by general tuberculosis, and usually 

 by death, whilst it is found that the cat and dog are much less 

 susceptible. Still more remarkable is the susceptibility of the 

 common house rat to anthrax, the bacillus of which is so stoutly 

 resisted by the white rat. Clearly some individuals, and even 

 species, seem to have the power of resisting disease germs — they 

 enjoy an immunity, or, in other words, are not susceptible. The 

 fact is important, and it is not surprising that many theories should 

 have been advanced to explain it. Not the least fascinating of 

 these theories is that which regards infection and resistance in 

 disease as a struggle between an attacking force of pathogenic, i.e. 

 disease-producing, organisms, and a defending army of cell elements. 

 This theory has long been held by various observers, but it is to 

 Professor Metchnikoft, a biologist, that we are indebted both for a 

 clear statement of the grounds upon which it is based and for obser- 

 vations which have made the acceptance of the theory possible. 



Metchnikoff shows that even in unicellular organisms, as the 

 amceba, a microbe in order to infect the Protozoon has to combat 

 the power possessed by the protoplasm of the latter of ejecting or 

 digesting the parasite. This is further and more completely illus- 

 trated by the phenomena observable in Infusorians. These may be 

 invaded by lower organisms, but it is only when they reach the 

 nucleus or nucleolus, that they have any chance of living and con- 

 quering. Hafkine introduced Paramcecia into capillary tubes con- 

 taining spores of a rod-like parasite, but although these spores were 

 swallowed and engulfed yet they could not combat the digestive 

 force of the protoplasm, and they were soon ejected like a foreign 

 body. If, however, the spores reached the nucleus or nucleolus, 

 neither of which has any digestive capacity, then they flourished 

 and germinated to the disadvantage of the Infusorium, which indeed 

 might die. On the other hand, the infested Infusorium might 

 survive and undergo division ; during division some of the parasites 

 escape from the nucleus into the surrounding protoplasm, and 

 coming under its influence are ejected, and this being repeated again; 

 and again, all the parasites may be expelled, the Paramcecium re- 

 covering completely. 



Thus, it seems quite possible that this digestive and expulsive! 

 power of protoplasm, is the defensive agent which protects thei 

 organism against invasion. In more complex multicellular organisms! 

 cells manifesting these powers are called Phagocytes. It is obvious! 

 that defending organisms must be capable of amoeboid movement) 

 They must be able to approach foreign bodies — bacteria, &c— iri 

 order to swallow and digest them, or in some cases, it may be, td 



