294 THE PROTOZOA 



which are about to reproduce themselves by some form of fission, 

 while the small forms are those which have resulted from a recent 

 act of reproduction. 



The multiplication of T. lewisi in the rat's blood takes various forms- 

 (Fig. 127). In some cases a trypanosome divides by equal binary fission 

 (C, D), but this is comparatively rare. More usually the fission is markedly 

 unequal, and of a multiple type. Small daughter-forms are split off from 

 large parent-individuals, and usually many at a time; the nucleus of the parent- 

 form divides several times, and subsequently the body divides into as many 

 portions as there are nuclei, thus producing rosette-like forms (Fig. 127, 

 F, G, H) in which the original parent can usually be distinguished by its long 

 flagellum from the small daughter-individuals with their flagella growing 

 out. The small forms are sometimes set free with a crithidial type of struc- 

 ture, the kinetonucleus in front of the trophonucleus (Fig. 127, L), and these 

 immature forms may proceed to reproduce themselves rapidly again by 

 either binary or multiple fission, in the latter case forming rosettes in which 

 no large parent-form can be distinguished (Fig. 127, K). 



A curious type of trypanosome found during the multiplication-period of 

 T. lewisi is a form with the posterior end prolonged to a great length, so 

 that it almost resembles a second flagellum (Fig. 127, E), and has sometimes 

 been mistaken for such. This form has been described by Lingard as a dis- 

 tinct species under the name T. longocaudense. These forms appear to arise 

 by binary fission (Fig. 127, J>) ; they are of constant occurrence and very 

 numerous at a certain stage of the multiplication-period. 



The multiplication of T. lewisi in the rat's blood is most active 

 from the eighth to the tenth day after infection, after which it is 

 on the decline and gradually ceases. The relative number of forms 

 of ordinary size increases steadily, while those of unusual dimen- 

 sions, whether great or small, become continually scarcer, until 

 about the twelfth or thirteenth day the trypanosomes, now usually 

 present in vast numbers in the blood, are of uniform size and 

 appearance, exhibiting, apart from occasional abnormalities, indi- 

 vidual variations only of a comparatively slight character ; and all 

 multiplication has ceased entirely, never to recommence in the 

 same host. The trypanosomes swarm in the blood of the rat for 

 a certain time, which varies in different cases, but is usually one 

 or two months. The infection of the rat is sometimes spoken of as 

 " acute " when the trypanosomes are multiplying, and as " chronic " 

 when multiplication has ceased, not, however, very well-chosen 

 terms, since the trypanosomes soon begin to diminish in number, 

 and finally disappear altogether ; sometimes the diminution is very 

 gradual and slow, sometime? it takes place with great rapidity. In 

 either case the rat gets rid of its infection entirely sooner or later, 

 without having suffered, apparently, any marked inconvenience 

 from it,* and is then immune against a fresh infection with this 

 species of trypanosome. 



* Instances are on record of lethal epizootics of rats ascribed to T. lewisi ; but 

 the proof that this parasite was really the cause of the disease is lacking. Under 

 normal circumstances rats show no perceptible pathological symptoms whatever 



