134 TEXT-BOOK OF BACTERIOLOGY. 



The most enthusiastic and ingenious supporter of this view is 

 Metschnikoff, who has developed it into a definite theory, which he 

 has endeavored to prove by direct experiments. Metschnikoff saw 

 that in certain daphniaceag a yeast fungus (blastomyces) which 

 had been received along 1 with food occasionally brought about a 

 general infection of the whole body. The vegetable parasite, boring 

 through the alimentary canal, found its way into the tissues and 

 multiplied within them. Frequently, however, the attack does not 

 end fatally, and in such cases Metschnikoff invariably observed 

 a remarkable phenomenon. 



If we examine a preparation of an infected water-flea strongly 

 magnified, we will usually notice without difficulty that a portion 

 of the sharp fungus-spore penetrates the intestinal wall. This part 

 looks somewhat degenerated, as if it had been gnawed; and further, 

 it will be seen to be surrounded by a throng of white blood-corpus- 

 cles, whose numbers will increase every moment and which will at- 

 tack the micro-organisms on all sides. 



It is they which, according to Metschnikoff, play the most im- 

 portant part in the whole process. As descendants of the medial 

 germinal layer, or mesoderm, they are closely related with the ele- 

 ments of which the digestive apparatus of all the higher animals 

 consists. They consequently possess the capacity of receiving and 

 devouring foreign bodies, and in the struggle between cells and 

 bacteria they make free and full use of their power. If they suc- 

 ceed in overpowering and destroying the yeast-fungus germ, the 

 organism is victorious and the animal is saved. If they fail to 

 destroy the germ, the foreign invaders are victorious and the scene 

 ends with the death of the animal. 



Generalizing his observations and their consequences and ap- 

 plying them to the human body, Metschnikoff came gradually to 

 the conviction that in every case of infection it is the white ele- 

 ments of the blood that, as scavenger-cells (phagocytes), have to 

 save the organism if they can. If bacteria attack any part of the 

 bocty, these cells, favored by their mobility, at once appear at the 

 place of danger and rush upon the invaders. If they are able to 

 make the latter innocuous no infection takes place; if, however, 

 these defenders of the organism struggle ineffectually and yield, 

 the enemies begin to multiply and spread themselves over the un- 

 protected domain. The latter is always the case when the attack- 

 ing bacteria are virulent to the animal attacked. If, for instance, 

 we inoculate a mouse or a Guinea-pig with full-strength anthrax 

 bacilli, there is no appearance of a reception of bacilli on the part 

 of the blood-cells. Yet such a reception does take place when we 



