DIGESTIVE ACTIVITY OF MESENCHYME. 107 



tioned, but they are very scarce. An endothelial origin of some 

 of the wandering cells cannot be excluded altogether since occa- 

 sionally small vessels are injured and their endothelial cells might 

 be transformed into wandering cells. Their number, however, 

 must be of small account indeed in comparison with those brought 

 in by the blood stream. 



2. Digestion of Injected Edestin. The process of digestion 

 is inseparably connected in our mind with a specialized system, 

 all of which is derived from the entoderm. This includes a spe- 

 cial cavity, in which the process is completed outside of cells and 

 tissues. A group of specifically differentiated secreting organs 

 are furnishing the enzymes, which, though of utmost importance 

 in digestion, still escape a more precise definition of their chem- 

 ical structure. We still judge of the presence of enzymes by the 

 result produced by them. Owing to the observation of results 

 obtained in the unicellular organism, we easily accord to the pro- 

 tozoa an intracellular power of digestion. In this case the diges- 

 tive ferments become active within a vacuole surrounded by 

 living cytoplasm, and while the conditions within this vacuole 

 bring about digestion of the ingested food (whether living or 

 dead) they do not affect in the same manner the cytoplasm of 

 the acting organism. 



It was Metchnikoff's merit to have pointed out in the multicel- 

 lular organism the role of phagocytosis with subsequent digestion 

 of the ingested material. The phagocytes described by him be- 

 longed to two classes distinctly and differently organized, the 

 polymorphonuclear leucocytes and the macrophages. The poly- 

 morphonuclear leucocytes did not require much interpretation, 

 but the macrophages became an object of numerous investiga- 

 tions. Their nature and origin are much disputed, possibly 

 because results obtained by study of definite cases and true, if 

 limited to these, were extended beyond the sphere of their control. 

 The macrophages are derived in turn from mesenchymal and from 

 endothelial cells. Blood stem cells (lymphoid hemoblasts) were 

 also seen to become macrophages. A special line of resting or 

 histiotopic wandering cells, eminently phagocytic, was observed 

 to develop from the mesenchyme in later embryonic stages. . The 



