THE PATHOGENIC PROTOZOA 143 



chsetes are not yet clearly determined. These questions, 

 which seemed of zoological and philosophical interest rather 

 than medical, have nevertheless come into the domain of 

 medicine since the discovery of the microbe of syphilis and its 

 treatment. From the study of the pathogenic protozoa there 

 have arisen many new ideas. 



In the study of bacteria the methods are their isolation and 

 pure culture, the study of their biochemical reactions, and of 

 experimental inoculations. Their physiology, i.e., the study of 

 their functions, takes precedence of the study of their forms, 

 i.e., their morphology. In the case of the protozoa, morphology 

 has the first place. It is no longer a question of describing 

 one cell, but a cycle of very dissimilar cellular forms with repro- 

 ductive phases, sometimes sexual, sometimes asexual. And it 

 is only at this price that certainty can be attained on the 

 methods of transmission of these microbes and on the basic 

 ideas for medicine and hygiene. Cultivation has been success- 

 fully accomplished only in the case of certain species, the 

 trypanosomes of the rat, amoebae in mixed culture, and 

 piroplasma, and it does not give the same help as do bacterial 

 cultivations. 



No intracellular protozoon nas yet been cultivated. 



It is the knowledge of the life-cycle which gives the key to 

 the transmission. The most complicated modes of transmis- 

 sion among the bacteria are very simple in comparison with 

 those of malaria and sleeping sickness. The living carriers of 

 certain bacteria, e.g., the rat-flea in the case of plague, appear 

 to be carriers pure and simple, i.e., possess practically the same 

 importance as the needle of a syringe or a lancet. The mos- 

 quito is more than a mere carrier of the haematozoon of 

 Laveran ; it is a second host and in it, and it alone, the parasite 

 accomplishes the sexual phase of its life-cycle. 



It was originally thought that the tsetse fly ( Glossina palpalis] 

 whose bite produces sleeping sickness was a simple carrier of 

 the virus and only remained infective for a few hours after 

 sucking it from the patient's body. But the recent experiments 

 of Kleine, confirmed by Bruce, have proved that the tsetse is 



