PHAGOCYTOSIS 275 



been reduced to small amorphous particles in this way, these are 

 seized by the ameboid cells, and intracellular digestion completes the 

 process which has been begun extracellularly. 



As we study the process among higher animals, it appears that, 

 among vertebrates, the intracellular methods of digestion have been, 

 at least for normal metabolism, entirely displaced by the extracellu- 

 lar as it occurs in the intestine, where solid particles are rendered 

 completely amorphous, dissolved, and reduced to a diffusible condi- 

 tion by the digestive juices before they are offered to the cells for 

 utilization. However, the capacity for intracellular digestion is not 

 entirely lost, and is retained of necessity in certain body cells. For 

 were there not such an emergency arrangement the body would lack 

 an available mechanism with which to meet such accidents as ex- 

 travasations of blood, or the entrance of bacteria and other foreign 

 solid particles into the tissues. It seems reasonable to classify both 

 the phagocytic action of body cells and the formation of antibodies 

 in the blood plasma, primarily as emergency devices for the diges- 

 tion of foreign materials both formed and unformed which, under 

 abnormal conditions, penetrate into the physiological interior of the 

 body (blood stream or tissue spaces), and must be disposed of. 



In the lowest animals the single cell is called upon to perform 

 all necessary functions. In the course of evolution, however, as the 

 body becomes more and more a community of many cells, a division 

 of labor takes place which is expressed morphologically in the differ- 

 entiation of tissues and organs, and physiologically in the adaptation 

 of individual tissue cells to the performance of specialized functions. 

 Nevertheless, it is necessary, both for certain normal processes, as 

 well as for provision against such complex emergencies as those 

 mentioned, that certain cells of the complex community should retain 

 the primitive abilities of the more independent cells of the lower 

 forms. Thus, among many animals, the phagocytic action of cells 

 performs definite services in the course of normal development. 

 This is seen most markedly in some insects (diptera) in which the 

 destruction of larval organs, useless to the adult animal, may be en- 

 tirely accomplished by the action of phagocytic cells, and a similar 

 process may accompany the transformation of the tadpole to the adult 

 in many amphibia. 9 In higher animals the removal of extravasations 

 of blood is accompanied by a train of occurrences which is readily 

 subjected to study. 10 In such cases the leukocytes rapidly enter the 

 area of extravasation and an engulfment of the blood cells occurs, 

 followed by a process of digestion entirely analogous to the digestion 

 of similar blood elements by the various forms of intestinal hem- 

 amebse. In the latter case it is a process of normal digestion, in the 



9 See Henneguy. "Les Insectes," Paris, 1904, p. 677. 

 10 Langhans. Virchow's Archiv, Vol. 49, 1870. 



