144. 



MICROBES AND TOXINS 



really a second host. It remains infective for about 24 to 48 

 hours after the moment of drawing infected blood, then for a 

 period of about 17 days it is non-infective, again becoming 

 infective for a period of about 60 days' duration. The trypano- 



some undergoes in it 

 an evolution with 

 sexual reproduction. 



The protoplasm of 

 protozoa appears to 

 possess faculties of 

 adaptation and varia- 

 tion much more exten- 

 sive than that of bac- 

 teria. Biologically 

 speaking it is a long 

 journey for a parasite 

 of the stomach of the 

 mosquito to reach the 

 blood of man. In the 

 successive phases and 

 habitats there are 

 forms and structures 

 so different that it re- 

 quires strict proofs to 

 convince one that it is 

 really the same species. 

 The Leishmania donovani of Kala-azar (a disease of India and 

 the Mediterranean) is in the intestine of a bug a flagellate 

 form : in man it is an intracellular form deprived of all 

 locomotory organs. The degradation resulting from para- 

 sitism has abolished in many forms the characteristic 

 structures and in many cases permits of only provisional 

 classification. 



These degradation phenomena are more striking among the 

 protozoa because we know on the other hand their complete 

 cycle. The bacteria, as we have seen, have reached in their 

 apparent simplicity perhaps the final stage of degradation, and 



FlG. 56. Glossina palpalis, the tsetse fly 

 which carries sleeping-sickness. 



