TRYPANOSOMA LEWISI 465 



Morphology.— The trypanosome form which is present in the blood 

 of the rat for the longest time is the one which occurs after the multipli- 

 cation phase (Fig. 197, 18-19, and Plate V., l, p. 456), and is generally 

 spoken of as T. lewisi, though this name applies to all stages of its develop- 

 ment in the rat and flea. The trypanosome as seen in the later stages 

 of an infection is a very characteristic organism. Very similar forms 

 occur in the blood of other small mammals, and they are often referred 

 to as being of the T. leivisi type. These forms (Plate V., l, p. 456) are 

 about 25 microns in length, and have a distinctly curved body which is 

 sharply pointed at its posterior end. There is a well- developed kineto- 

 plast situated at some distance from the pointed posterior extremity. 

 The nucleus is definitely anterior to the central point of the body. The 

 undulating membrane is not markedly convoluted, the axoneme along 

 its border running a fairly straight course. There is a well-developed 

 flagellum beyond the anterior extremity of the organism. The curved 

 body, the sharp posterior end, and the excentric position of the nucleus 

 give these forms of T. leivisi a very characteristic appearance. Apart 

 from the nucleus and kinetoplast, the cytoplasm of the trypanosome is 

 usually free from granules, but certain structures not always visible have 

 been described as of occasional occurrence (see p. 323). 



During the multiplication phase of T. lewisi, which commences shortly 

 after inoculation, the trypanosomes which occur in the blood-vessels 

 exhibit an extreme degree of polymorphism. There are large broad 

 trypanosomes with prolonged and pointed posterior ends with their 

 kinetoplasts adjacent to the nucleus, very much smaller forms of the same 

 type, and small round forms provided with flagella. Types intermediate 

 between all these also occur. These variations are best comprehended 

 by reference to the figure (Fig. 197, 1-15). 



In the living condition the typical trypanosomes are exceedingly active, 

 and dash about with great energy amongst the red blood-corpuscles 

 with flagellar end in front, quickly passing from one microscopic field to 

 another. The large multiplication forms and others seen in the early 

 stages of an infection are much less motile. 



Multiplication. — As already noted, the multiplication phase is of short 

 duration, and is characterized by the marked polymorphism of the try- 

 panosomes. It may be said to commence with the large broad trypano- 

 somes, which measure at least 35 microns in length and have the kineto- 

 plast near the nucleus (Fig. 197, 1-3). Division of the kinetoplast takes 

 place, followed by that of the nucleus. From the daughter kinetoplast 

 is formed a new axoneme, which does not grow to the length of the original 

 one, so that a short undulating membrane is formed. The cytoplasm then 

 divides between the flagella, and a small daughter individual is separated. 

 I. 30 



