THE CELLULAR MECHANISMS OF DEFENCE 



1023 



larva consists of a sack which is involuted at one extremity to form the 

 alimentary cavity, while the mesoblast is represented by amoeboid 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 phagocytosis is not always 

 successful in maintaining the health or life of the host. Thus if the spores 



end. 



FIG. 480. 1, gastrula stage of starfish embryo, with a foreign substance, pi, in its 

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

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

 (highly magnified). (After METCHNIKOFF.) 



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 to the death of the host. The same process of phagocytosis may 

 be studied in its simple form by injuring or infecting some tissue which 

 is free from blood-vessels. Thus the tail fin of an embryonic axolotl may 

 be cauterised with silver nitrate, or a small quantity of fluid containing 

 carmine granules may be introduced by means of a hypodermic syringe. In 

 either way a certain number of cells are destroyed and the dead tissue there- 

 upon acts as a foreign body. As a result the wandering mesoderm cells or 

 leucocytes move from the surrounding tissues towards the seat of the injury, 

 and the day after the injury has been inflicted a collection of leucocytes 

 can be seen, many of which contain particles of carmine or debris of the 

 destroyed tissue which they have taken up. The cells finally wander away 

 from the part, and the destruction is made good by the proliferation of the 



