THEORIES OF IMMUNITY. 



kingdom (except protozoa). Considering the 

 ectoderm as the source of the covering of all 

 primitive polycellular animals, and the entoderm 

 as the source of the organs of digestion, the 

 part of the mesoderm remained a mystery until 

 certain of his studies on sponges led him to 

 think that perhaps this layer might have acted 

 in the hypothetical primitive animals as a mass 

 of digestive cells, in every respect like those of 

 the entoderm. Such a theory necessarily at- 

 tracted his attention to the property that these 

 mesodermic cells have of seizing upon foreign 

 bodies. 



The fact had been known for a long time, both 

 in respect to the power of the white corpuscles 

 of the vertebrates to seize upon foreign bodies 

 and of the ameboid cells to seize upon particles 

 of coloring matter. But no one had looked 

 upon this property as one of digestion, and it 

 had even been considered as simply a passive 

 action. Metchnikoff's studies upon sponges and 

 some of the simpler forms of sea animalculse 

 convinced him that the presence of foreign 

 bodies in the ameboid cells of the mesoderm 

 should be interpreted as an active englobement, 



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