TRYPANOSOMA LEWISI 655 



suggests the possibility of a sexual element iu a trypanosomal 

 cycle, there is at present no definite proof that such a stage 

 has ever been observed. The occurrence of cyclic phases does 

 not necessarily involve the interposition of conjugation. 



It has been found possible to cultivate a great number of the 

 trypanosomata outside the bodies of their natural hosts, the first 

 work having been done by Novy and MacNeal, who introduced 

 a special medium for the purpose (p. 46). In cultures, the 

 organisms may divide longitudinally, as seen in the blood, or 

 crithidial or Leishmania forms may result, the former being 

 often arranged in rosettes containing a large number of indi- 

 viduals with their flagella pointing in one direction. A fresh 

 infection may sometimes be originated by introducing such 

 cultures into suitable animals. 



While many trypanosomes give rise to serious disease, in 

 many cases a heavy infection may occur without the animal 

 suffering any apparent inconvenience, and a form producing 

 disease in one species may be present in considerable numbers 

 in another species without causing any pathogenic effects. 



We now pass to consider in detail some of the more important 

 trypanosomes. 



Trypanosoma lewisi. — This trypanosome, which Lewis in 1878 de- 

 scribed in India, has been found to be very common in the blood of rats 

 all over the world, though the percentage of animals affected varies in 

 different localities. The organism has no importance from the stand- 

 point of human pathology, but the condition in the rat is of great 

 interest, as, though the infection runs a very definite course, it is very 

 rarely fatal ; in fact, most observers have been unable to produce death 

 by infecting even large series of animals. A fatal issue may, however, 

 occur in young individuals, especially when these are infected with 

 strains of the organism imported from other localities. The trypanosome, 

 which is actively motile, is of ordinary length but is somewhat narrow, 

 and its protoplasm does not contain any granules. It multiplies by 

 fission, of which Laveran describes two varieties. In one, the organism 

 splits ' longitudinally and gives rise to smaller individuals than the 

 parent. In the other, the trypanosome loses its ordinary shape and 

 becomes more oval : nuclear division, which is often multiple, then takes 

 place, and on subsequent division of the protoplasm a number of small 

 flagellate organisms result ; these last may attain the full form and 

 size before dividing again, or they may divide when still small. When a 

 rat is infected by injection into the peritoneum, active multiplication goes 

 on in the cavity for a few days and then comes to an end. Very soon 

 after infection the organisms begin to appear in the blood and there 

 rapid multiplication occurs, the extent of which is sometimes so great 

 that the trypanosomes may seem to equal the red blood corpuscles iu 

 number. The animal usually shows no symptoms of illness. The 

 infection "oes on for about two months, and then the organisms gradually 

 disappearfrom the blood. In the great majority of cases the rat is now 

 immune against fresh infection. If trypanosomes be introduced into its 



