96 TRYPANOSOMES AND SLEEPING SICKNESS 



form, differing from the preceding in that the parabasal body 

 has moved back to near the middle of the body, and the flagellum 

 is connected with the body for half its length by an undulating 

 membrane (Fig. 18B). This type is a very common develop- 

 mental phase in nearly all trypanosomes, but it is also the adult 

 condition of many insect parasites. Finally there occurs the 

 fully-developed trypanosome form (Fig. ISA), apparently es- 

 pecially adapted in form and structure for life in vertebrate 



bodies. The method of develop- 

 ment of this form from a crithidial 

 type can easily be seen from Fig. 18. 

 Only the first or Leishmania form 

 and the last or trypanosome form 

 normally occur in vertebrate bodies, 

 though all of the four types are 

 found in the digestive tracts of in- 

 vertebrates. The fact that some 

 flagellates never develop further 

 than the Herpetomonas form, and 

 others never further than the Crith- 

 idia form, makes a study of this 

 group of Flagellates very confusing, 

 since when a Herpetomonas or crith- 

 idial type is found in an insect gut 

 it is very difficult if not impossible 



FIG. 19. Trypanosoma rhodesi- to sa y whether it is an adult animal 

 ense, from blood of monkey inocu- which never undergoes any further 



lated from case of human sleeping j i i j i 



sickness. Note posterior position of development or is only a develop- 

 nucieus in short blunt forms, espe- mental phase of a trypanosome of 



cially in lower figure. X 2000. n^^u,. + " l 



(After Kinghom and Yorke.) a vertebrate animal. 



It is often very difficult to dis- 

 tinguish different species of trypanosomes; of over 70 known 

 species only a few can be distinguished on morphological grounds. 

 Average size, position of nucleus and parabasal body, length of 

 snout, and presence or absence of a free flagellum are sometimes 

 useful in identifying them. More reliable, however, are their so- 

 called " biological characteristics," such as pathologic effects on 

 different animals, susceptibility of different hosts, the effect of 

 serum immune to certain species, and the " cross-immunity re- 

 actions." The last is the most certain method. Thus, if an 



