PHAGOCYTOSIS 241 



substances are absorbed from the alimentary canal of the lower 

 invertebrates by a process of intracellular digestion, and by that 

 alone. Thus, when the alimentary canal of a planarian (Dendro- 

 ccelum lacteum, an animal resembling the liver-fluke in its general 

 anatomy) is filled with blood, the latter is found to undergo the 

 changes in colour familiar in bruises, and this is due to the fact 

 that the blood has been taken from the alimentary canal by the 

 cells lining it, and there has undergone the digestive changes. 

 These processes can readily be traced under the microscope, and 

 the blood-corpuscles can be seen, at first embedded in protoplasm 

 and of normal contour, and later enclosed in vacuoles and of altered 

 shape. Complete digestion takes several days, and every stage of 

 the process is easy to watch. Absorption of organized food particles 

 takes place from the alimentary canal in exactly the same way 

 in actinians, molluscs, and many other lower animals, and in 

 many of the cases which have been investigated the vacuoles are 

 found to contain a digestive ferment allied to trypsin or pepsin. 

 In higher animals this process of intracellular digestion does not 

 occur, or only to a slight extent (in the case of fats), the animal 

 finding it more advantageous to secrete the digestive juices into 

 the alimentary canal, and to absorb the products of their action 

 therefrom in a state of solution. 



Absorption of particulate substances which have gained access 

 to the tissues takes place, to a very large extent, in a method 

 entirely similar. Thus Metchnikoff showed that when bird's 

 corpuscles are injected into the tissues of the larva of the cock- 

 chafer, or into the snail, earthworm, the peritoneal cavity of the 

 goldfish, etc., the process is also intracellular and entirely similar 

 to those which occur when these corpuscles are injected into the 

 alimentary canal of the planarian. In most cases the corpuscles 

 are absorbed by the leucocytes, in others by the cells of the part 

 (such as the endothelial cells lining the peritoneum) ; but in all 

 cases there is the same vacuolation, the same series of changes 

 in the ingested corpuscles, and the same final result. We shall 

 not be far wrong in associating the absorption of these corpuscles 

 in a very close way with processes of digestion and nutrition. 



The French school have studied these processes of absorption 

 of particulate bodies from the tissues in a very complete manner, 

 and have shown beyond dispute the importance of phagocytosis 

 in this respect. Thus particles of carbon in the lung, the granules 

 of pigment left after interstitial haemorrhages, etc., are all ingested 



16 



