Immunity against pathogenic micro-organisms 131 



pneumonia. It is unnecessary to multiply examples to obtain a 

 general proof that natural immunity against micro-organisms cannot 

 be explained by the incapacity of these pathogenic agents to live in [139] 

 the fluids of the refractory organism. 



We must, however, ascertain what takes place in resistant animals 

 inoculated with micro-organisms. Here, again, it is preferable to 

 begin with the lower animals of simple organisation. We have already 

 seen that examples of natural immunity are not rare in the Inver- 

 tebrata. When engaged in the study of the disease found in 

 Daphniae, small Crustacea so common in fresh water, I was able 

 to show that the special Blastomycetes which cause it meet with 

 a vigorous resistance on the part of the organism. As the Daphniae 

 are small, transparent, and consequently easily observed under the 

 microscope, I was able without difficulty to establish the main 

 phenomena observable in these organisms. I can be the more brief 

 in describing these phenomena of resistance as, in addition to de- 

 voting a special memoir to the Daphnia disease 1 , I have, in my 

 Lectures on Inflammation (pp. 97 103) 2 , described at some length 

 the reaction of their organism to the Monospora. It is nevertheless 

 necessary that I should recall, very briefly, the mechanism by which 

 these small crustaceans secure immunity. 



The spores of the parasite very delicate and rigid needles are 

 swallowed with the food. By means of their sharp points they 

 perforate the intestine and penetrate into the body cavity, full of 

 blood, where they find themselves exposed to the attacks of leuco- 

 cytes. These leucocytes, guided by their tactile sense, gather around 

 the foreign body, ingest it completely and destroy it. It is remark- 

 able that the spore, which is furnished with a very resistant 

 membrane, once in the interior of the mass of leucocytes, undergoes 

 modifications which afford evidence of the presence in these cells 

 of an extraordinary digestive power. The surface of the spore, from 

 being smooth and regular, becomes pitted and sinuous, the spore 

 breaks up into fragments and is reduced to a mass of debris which, 

 in the form of brown granules, remains indefinitely in the contents 

 of the leucocytes. From this it is evident that these phagocytes 

 must produce a ferment which is capable of digesting the cellulose 

 or analogous substance which forms the membrane of the spore. 

 Unfortunately, the small size of the Daphniae, so useful for the 



1 Virchow's Archiv, Berlin, 1884, Bd. xcvi, S. 177. 



2 [English translation, pp. 8386.] 



92 



