THE CELLULAR MECHANISMS OF DEFENCE 1145 



similar in character to the amoeba. They have the power of extruding 

 pseudopodia, of wandering from place to place, and of englobing and 

 digesting particles of food from bacteria with which they come in con- 

 tact. On account of these latter properties they have been called 

 by Metchnikoff phagocytes, and the whole process by which foreign 

 material or the animal's own dead tissues are got rid of is spoken of 

 as phagocytosis. The process can be well studied, as has been shown 

 by Metchnikoff, in the sponge or in the larva of the echinoderm. At 

 one stage in the development of the latter the larva consists of a sack 

 which is involuted at one extremity to form the alimentary cavity, 



end. 



FIG. 481. 1, gastrula stage of starfish embryo, with a foreign substance, pi, in its 

 body cavity ; end, endoderm ; ect, ectoderm ; mes, wandering mesoblastic 

 cells. 2, the foreign body of 1, surrounded by a plasniodiurn of phagocytes 

 (highly magnified). (After METCHNIKOFF.) 



while the mesoblast is represented by amo3boid cells suspended in a 

 semi-liquid substance filling the body cavity. If a particle of foreign 

 substance be introduced into the body cavity the wandering mesoderm 

 cells collect round the particle and fuse into plasmodial masses, thus 

 forming a wall, as it were, around it. If bacteria be introduced, the 

 phagocytes may be seen to adhere to and ingest the still living bacteria, 

 which are then rapidly digested and destroyed. A similar process 

 may be observed in the transparent crustacean known as the water- 

 flea (Daphnia), and here it may be noted that the "process of phago- 

 cytosis is not always successful in maintaining the health or life of the 

 host. Thus if the spores of a yeast-like organism, the Monospora, be 

 introduced into the body cavity of Daphnia, the leucocytes may, if 

 the spores be few in number, lay hold of the latter and digest them. 

 If the spores be in excess the phagocytes may fail to ingest them 

 or may indeed be destroyed as soon as they approach them. In this 

 case the spores germinate, fill up the body cavity, and finally lead 



