PROPERTIES OF SERUMS. 9 



injection of the killed bacteria, or, in some cases, 

 of their non-living soluble products. 



Metchnikoff may be credited with having first 

 offered a plausible explanation of natural resist- 

 ance, founded on observation. As a zoologist he 

 had studied the subject of intracellular digestion 

 in the lower animals, and it was while working on 

 this problem that he observed the fate of a yeast 

 fungus (Monospora) , which caused epidemics 

 among the daphnia, small, transparent animals 

 with which he was working. Near the alimentary 

 tract, which was the infection atrium, some large 

 mesoblastic cells, which are perhaps analogous to 

 the white blood cells, were seen to ingest the para- 

 sites and dissolve them. If this took place to a 

 sufficient extent the animals recovered; if, how- 

 ever, the infecting organisms were too numerous 

 or the reaction on the part of the animal insuffi- 

 cient, the body became overwhelmed with para- 

 sites and death resulted. Since that time Metch- 

 nikoff has evolved his well-known theory of pha- 

 gocytosis as the essential factor in both natural 

 and acquired immunity, a theory which Pasteur, 

 in his later years, looked on with favor. We may 

 speak of this as the cellular theory of immunity; 

 a theory which has had to undergo important 

 modications in order to bring it into accord with 

 new facts. 



Considering that natural or acquired immunity i, 1V estis- 

 must exist because of certain qualities of the body p^e/tie* 

 cells, or of the body fluids, or possibly of both, of serums. 

 investigators began to make analyses of the tis- 

 sues; and of all the analyses, that which we may 

 term the biologic has been the most fruitful. In 

 this case biologic analysis means the detection of 



