i8 94 ] THE BRITISH NATURALIST. 205 



retreat from antagonistic germs. The attraction which draws a 

 phagocyte towards a body suitable for its action is known as positive 

 chemiotaxis, whilst the repelling force which causes a leucocyte to 

 retire from an enemy is named negative chemiotaxis. 



Phagocytes, then, may be defined as all elements which manifest 

 chemiotactic properties and which possess the power of englobing 

 and digesting substances and either absorbing them entirely or 

 rejecting the residue. 



In those animals which possess formed elements in the blood, 

 some of the leucocytes — though not all of them — are phagocytic, 

 but endothebal cells and mobile connective tissue cells also supply 

 phagocytes. 



The phenomena of Phagocytosis in themselves are profoundly 

 interesting to the naturalist, and in view of the theory of Metchni- 

 koff — that in them we have the explanation of the power of re- 

 sistance to disease germs which constitutes immunity — they are no 

 less so to the pathologist. The theory is not free from objections, 

 but it is not my object in this brief sketch to discuss the various 

 objections to it, or the alternative theories which have been advanced, 

 so much as to draw attention to the subject of phagocytes and 

 their influences. 



Nothing could be more suggestive or more illustrative than the 

 record of Metchnikofi's observations upon Crustacean's, many of 

 which are subject to infectious diseases. The most interesting 

 instance is furnished by the Daphniae, which not unfrequently are 

 invaded by a yeast-like fungus multiplying by budding (monospova 

 bicuspidata) which may be found as long spores enclosed in a capsule 

 in the body cavity of the Crustacean in large numbers (Plate X., 



Fig- 3)- 



The Daphniae devour the spores, and are thus infected through 

 their alimentary canal. Losing its capsule, a spore punctures the 

 wall of the canal and reaches the body cavity. It is then immediately 

 attacked by leucocytic phagocytes carried by the blood stream, a 

 group of which collects round the spore. Under their influence the spore 

 is destroyed and breaks up into a mass of granules (Plate X., Fig. 4), 

 having no resemblance to the original spore, and that this is due 

 to phagocytic influence is proved by the fact that when only part 

 of the spore is protruded into the body cavity, the rest, remaining 

 in the wall and so being protected, remains normal. 



The destruction of the spores may be complete ; on the other 

 hand the spores may prove too strong or too numerous, and some 

 begin to germinate and give rise to budding conidia. They then 

 usually gain the victory, and the leucocytes disappear. As Metch- 

 nikoff concludes, "There can be no doubt that the whole history of 



