132 Chapter VI 



direct observation of the phenomena of immunity, presents an insur- 

 mountable obstacle to the study of its leucocyte ferments, especially 

 in vitro. 



[HO] The destruction of the spores of the parasite by the leucocytes 

 secures to the Daphnia a real immunity. Of a hundred Daphniae 

 taken in my aquarium and carefully examined under the microscope, 

 fourteen only were found to be infected by the budding conidia of 

 the parasite, whilst fifty-nine of the others contained the remains of 

 spores that had been destroyed by the phagocytes. When transferred 

 to pure water containing no new source of contagion, these Daphniae 

 flourished and lived a normal life, giving birth to a numerous progeny. 



The immunity of the Daphnia, due to the intervention of 

 phagocytes, is an example of natural, individual immunity. It is 

 not the specific or racial possession of these Crustacea, for when 

 the leucocytes do not seize the spore, at once, on its penetration into 

 the body cavity, it commences to germinate and gives rise to a whole 

 generation of budding cells. These cells, then, secrete a poison which 

 not only repels the leucocytes, but kills and completely dissolves 

 them. Under these conditions the Daphnia is disarmed ; the 

 parasites grow in the organism, deprived of its arm of defence, as 

 in a culture tube, and the animal rapidly succumbs. 



Since I first observed this struggle between the Daphnia and its 

 parasite, some eighteen years ago, no other example has been found 

 that is so easily observed and so demonstrative of the protective 

 action of phagocytes in an animal that can be kept under observation, 

 alive, under the microscope. Cases, however, are not wanting in the 

 Invertebrata in which the different phases of this struggle may be 

 observed with sufficient accuracy to warrant the conclusion that in 

 these cases also the phenomena are analogous to those observed in 

 the case of the Daphniae. 



It has already been stated in Chapter in. that the larvae of the 

 rhinoceros beetle (Oryctes nasicornis), although very sensitive to the 

 cholera vibrio, are very refractory to anthrax and diphtheria. In 

 order that we may obtain some idea of the mechanism of this im- 

 munity let us inject into the body cavity of these large white grubs 

 a trace of anthrax culture. In the blood, drawn off* the following 

 morning, the injected bacilli are found, not in the plasma, but inside 

 many of the leucocytes. Here there has occurred, as in the Daphnia, 

 an ingestion of the parasites which have then been destroyed by the 

 intracellular digestion of phagocytes. The process is the same, then, 



