PHAGOCYTOSIS 239 



observation. Leucocytes, it may be pointed out, are cells which, 

 from their isolation from one another, power of independent move- 

 ment, etc., are closely analogous with amoebae and other lowly 

 organized protozoa. 



Metchnikoff found that daphnia is subject to a disease due to 

 the invasion of its body-cavity by the spores of a yeast the 

 monospora and that if these spores gained access in large 

 numbers they multiplied, formed into mature organisms, and 

 finally killed their host. When, however, few spores gained 

 access, he found that the daphnia's leucocytes approached them, 



FIG. 46. AN AMCEBA WHICH HAS INGESTED NUMEROUS SPECIMENS OF 

 MICROSPH^RA. (Metchnikoff.) 



a, a. Vacuoles. 



formed a wall round them, and finally digested and destroyed 

 them. It is obvious, therefore, that the immunity of these 

 animals is relegated, so to speak, to its leucocytes. If these are 

 efficient, the animal is preserved from its invaders; whereas, 

 if they make default, the latter multiply, and bring about the 

 lethal issue. 



MetchnikofFs experiments were by no means confined to the 

 action of the leucocytes on bacteria. They included a careful 

 and exhaustive study of the method of absorption of all manner 

 of particles, organized and unorganized, in the tissues and body- 

 cavities of animals of all positions in the animal world, and they 

 proved to the full the importance of phagocytosis and intra- 

 cellular digestion, and one of the chief if not, indeed, the only 



