Immunity against pathogenic micro-organisms 173 



no longer yield growths. On the other hand Curtis' pathogenic 

 yeast-cells resist the action of the phagocytes for a much longer time. 

 After a period of phagolysis in the peritoneal cavity, the leucocytes 

 that have just arrived in large numbers begin to seize the yeast-cells. 

 Usually several macrophages fuse around the same yeast globule form- 

 ing a very characteristic kind of rosette. Sometimes the macrophages 

 run together to produce a giant cell, whose centre contains the 

 yeast-cell. This latter defends itself against phagocytosis by secreting 

 a fairly thick membrane. The struggle between the two living 

 elements is a fairly prolonged one; 24 to 48 hours after inoculation all 

 the yeasts are surrounded by phagocytes, amongst which microphages 

 are exceptional. But the parasites remain alive for 4 6 days after 

 their injection into the peritoneal cavity, as proved by the cultures that 

 are obtained from the exudation when the fluid is " seeded " out. It 

 must be concluded, therefore, that the yeast-cells were surrounded by 

 the phagocytes whilst still presenting all the signs of life. Skchiwan 

 was no more successful than Schattenfroh in demonstrating any kind 

 of microbicidal action of the fluids on the Blastomycetes. 



There is, consequently, no doubt whatever that the resistance of 

 the animal organism against yeasts follows the same rules that hold 

 in the defence against bacteria. 



The animal micro-organisms are much rarer in infective diseases 

 than are the microphytes; moreover the impossibility of obtaining 

 cultures of them renders their investigation much more difficult. 

 Yet there exist facts that are capable of affording us information as 

 to the means made use of by the refractory organism against certain 

 parasitic Protozoa. Amongst these latter the Trypanosomae play a 

 most important part. One species of this genus (T. lewisi) produces 

 an infective disease in rats, especially in the grey rat (31 us decu- 

 manns), the blood of these rodents often containing a very large 

 number of them, whilst the small flagellated organisms flourish well in 

 the serum prepared from the blood of affected animals. Laveran and [184] 

 Mesnil 1 , in their studies on the Trypanosomae, injected defibrinated 

 blood containing numerous Trypanosomae into the peritoneal cavity 

 of guinea-pigs, w r hich exhibit a natural immunity against this parasite. 

 The parasites remained alive for some days and then disappeared 

 completely. Here again it is the phagocytes of the peritoneal exuda- 

 tion which rid the animal of the Trypanosomae by ingesting them. 

 Laveran and Mesnil were able, by the examination of hanging drops 

 1 Ann. de I'Inst. Pasteur, Paris, 1901, t xv, p. 673. 



