Summary 547 



organisms, e.g. the larvae of flies, the digestive juices are powerless 

 to destroy them. Nevertheless, there are organisms which feed 

 exclusively, or almost exclusively, on bacteria and which are quite 

 capable of digesting them. These are the Protozoa, such as the 

 Amoebae and certain Infusoria, which, without any trace of a [571] 

 digestive tube, easily bring about this result. Amoebae can be 

 grown on the surface of agar by taking care to sow along with them 

 bacteria for their nourishment. It is only necessary to give them 

 a single species of micro-organism, and this may be selected from 

 the pathogenic forms, such as the cholera vibrio or the BaciUus coli. 

 The Amoebae ingest a number of these bacteria in the living state. 

 They then kill them and digest them in their digestive vacuoles 

 which contain, along with a little acid, a ferment belonging to the 

 trypsin group, the amoebodiastase. 



The bodies of lower and higher animals, alike, are very rich in 

 elements which closely resemble the Amoebae. Sometimes these are 

 to be found in the epithelial cells of the digestive canal which put 

 out protoplasmic processes for the purpose of seizing food and trans- 

 ferring it to their interior, where it is submitted to the action of 

 digestive ferments. Sometimes they are the cells disposed between 

 the body wall and that of the intestinal canal, which float freely 

 in the fluids of the body or are more or less fixed in the interstitial 

 tissue. The animal kingdom presents a great variety of these 

 amoeboid elements, known under the general name of phagocytes 

 (cells capable of devouring solid bodies). One of the most primitive 

 arrangements of phagocytes is met with in Ascaris and its allies 

 belonging to the group of the Xeniatoda. All the organisation 

 that these round worms possess consists merely of four, or a few 

 more, enormous cells attached to the body wall. These are phago- 

 cytes which push out processes of enormous length, capable of 

 exploring the whole of the internal cavity of the body. 



The majority of phagocytes circulate in the lymph and blood and 

 pass into the exudations. These white corpuscles have a comparatively 

 uniform structure in the Invertebrata and present themselves as small 

 cells with a nucleus and a protoplasm capable of amoeboid move- 

 ments. In the Vertebrata we meet with two great categories of white 

 corpuscles, of which one group resembles those of the Invertebrata 

 in that they also possess a single large nucleus and an amoeboid proto- 

 plasm. These are the macrophages of the blood and of the lymph, 

 and are intimately connected with the macrophages of such organs as 



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