PHYSIOLOGICAL 435 



tion of Nematode worms and Lancelets (Amphioxiis), there are 

 wandering amoiboid cells which are able to migrate from place to 

 place within the body. Like Amoebae they are able to engulf smaller 

 organisms and particles, and in some cases to digest them. These 

 are the phagocytes, or devouring cells. They often occur in large 

 numbers, and may be regarded in fact as mobile tissue. In Vertebrates 

 they are specialised white blood corpuscles (leucocytes), but they 

 are able to pass through the walls of the capillaries and to move 

 among the other cells of the body. Yet they are of older origin than 

 the blood, for they occur in animals like sponges and jelly-fishes 

 which have not attained to a blood-vascular system. They may be 

 thought of as body-cells which have retained the mobility and 

 plasticity of Amoebae. 



In 1862 Haeckel observed that grains of indigo injected into a 

 mollusc were soon taken in by amoeboid "colourless corpuscles". 

 The hint supplied by this suggestive fact was followed by others; 

 but it is to the credit of Metchnikoff that he recognised the important 

 role of these cells as not merely engulfing irritant particles, but 

 destroying them when digestible, thus waging war against intruding 

 microbes and parasite-germs. In his Lectures on the Comparative 

 Pathology of Inflammation (trans. London, 1893) he traced through 

 the animal kingdom the devouring function of the wandering 

 cells to which he had given the name of phagocytes. This highly 

 important conception we must therefore outline with special 

 emphasis on the internal defence of the animal bod}^ 



The simplest conditions are seen in the Protozoa, for there the 

 whole organism may act as a phagocyte, engulfing and digesting 

 microbes. This is particularly true of the more or less Amoeboid 

 Protozoa — the Rhizopods. The experiment has been made of intro- 

 ducing virulent bacteria into a drop of water containing amoebae, 

 with the result that many of them were ingested and destroyed. In 

 certain cases the bacteria or microbes are actually avoided ; for some 

 Protozoa exhibit a sensitiveness (chemotaxis) to particles with 

 which they are not in actual contact. Thus a Myxomycete will creep 

 towards a drop of decoction of dead leaves and away from one of 

 a salt solution ; it will "prefer" a nutritive fluid which is not swarming 

 with bacteria to one that is foul with them. It may be here noted 

 in passing that many Protozoa are quick to repair injuries and to 

 regrow lost parts, two processes in which, in Metazoa, the wandering 

 phagocytes often take a constructive share. But the most important 

 point is simply that the phagocytes which form part of the body of 

 a Metazoan are cells that retain the activities of Amoebae, and not 

 only as regards locomotion, but in their ingestive and digestive 

 powers as well. 



In Sponges there is often an avoidance of biggish intruders 

 by the simple expedient of closing the larger pores or oscula — an 



