IMMUNITY AND SUSCEPTIBILITY. 65 



against the streptococci. In this manner the battle con- 

 tinues, the cells now being obliged to yield to the bacteria 

 and the patch spreading, while the cells subsequently re- 

 inforce and destroy the bacteria, so that the disease comes 

 to a termination. 



Metchnikoff introduced fragments of tissue from ani- 

 mals dead of anthrax under the skin of the back of a frog, 

 and found it surrounded and penetrated by leucocytes con- 

 taining many of the bacilli. 



It need scarcely be pointed out that a loophole of doubt 

 exists in all these illustrations: the bacteria may have been 

 dead before the cells ingested them, and the phenomena of 

 digestion and destruction which have gone on in their in- 

 teriors may have been exerted upon dead bacteria. To the 

 relapsing- fever illustration we may take exceptions, and 

 state that the apyrexia may have marked the death of 

 the spirilla, which were taken up by the leucocytes only 

 when dead. In the erysipelas illustration the streptococci 

 remote from the centre of the lesion may have met from 

 the body-juices or some other cause a more speedy death 

 than that from the digestive juices of the leucocyte. 



Metchnikoff, however, is prepared to show us that the 

 leucocytes do take up living pathogenic organisms. He 

 succeeded in isolating two leucocytes, each containing an 

 anthrax spore, and conveying them to artificial culture- 

 media, where he watched them. The new environment 

 being better adapted to the growth of the spore than for 

 the nourishment of the leucocyte, the latter died, and 

 the spore developed under his eyes into a healthy bacillus. 

 Seeing that the animal cells take up bacteria, and seeing 

 that the ameba can ingest and digest "threads of lepto- 

 thrix ten times as long as itself," we need only put two 

 and two together to see that Metchnikoff 's theory rests 

 upon a very substantial foundation. The more virulent 

 the bacteria, the less ready the leucocytes are to seize 

 them. The more immune the animal, the greater is the 

 affinity of the leucocyte for the bacteria. 



The organisms which are seized upon by the leucocytes 



