ORGANISMS WHICH PRODUCE DISEASE 85 



and is found in the stools and in the walls of the intestine and 

 also in the liver-abscesses so often present in cases of tropical 

 dysentery (see Frontispiece, fig. 21). The parasite may also 

 find its way into blood-vessels. It measures 25 to 30 ft in 

 diameter, and in one stage is actively amoeboid, i.e. moves and 

 captures its food by the projection of pseudopodia or projections 

 of its protoplasm, especially of an outer clear layer known as 

 the ectoplasm. Within this is the endoplasm, which usually 

 has a peculiar "vacuolated" appearance, and in which is situated 

 a rounded nucleus which is poor in chromatin or colouring 

 matter and therefore difficult to stain. The parasite feeds upon 

 leucocytes, red blood-corpuscles, and other cells of its host, and 

 upon starch-granules, food-debris, and bacteria in the intestine. 

 It multiplies by fission, and by simple and multiple budding. 

 When in unfavourable surroundings, it becomes rounded and 

 enclosed in a thick wall or capsule, within which spores are 

 formed. These pass out with the faeces and carry the infection, 

 as has been proved by experiments on the cat. It has also been 

 successfully cultivated artificially in association with intestinal 

 bacteria upon which it feeds. 



Trypanosomes belong to the class Mastigophora or 

 Flagellates (see p. 14), and are parasitic in the blood of many 

 animals such as mammals, birds, amphibians, and fishes. Try- 

 panosoma lewisi is very common in the rat, and T. bmcei is 

 the cause of Nagana or Tse-tse Fly Disease, a terrible scourge 

 of cattle and horses in certain parts of Africa. Numerous other 

 forms cause various diseases in horses and cattle and other 

 animals in many countries of the world, but in man the most 

 serious form of disease so produced is Sleeping Sickness caused 

 by Trypanosoma gambiense. 1 This is a small eel- or fish- 

 like, actively motile parasite (see Frontispiece, fig. 22) which, 

 under the microscope, can be seen swimming with a wriggling 

 movement and also propelled by means of a fin-like undu- 

 lating membrane along one side of its body. This membrane 

 is prolonged at the anterior end into a whip-like flagellum. 

 The organism is about 20 p or more in length, and it 

 multiplies either by direct fission or by a complicated form 

 of sexual reproduction, for the completion of which a biting 

 fly, Glossina palpalis or some of its relatives, is required, these 

 flies constituting the carriers of the infection among man and 

 animals. In Uganda about a quarter of a million natives more 

 than two-thirds of the entire population have perished of this 



1 Trypanosoma rhodesiense is another closely allied organism which 

 produces practically identical results. 



