DEVELOPMENT IN MAN 



101 



Glossina morsitans to become infective varies from 11 to 25 days, 

 but under unfavorable conditions the parasites may remain in 

 the fly in an incomplete stage of development for at least two 

 months. A temperature between 75 F. and 85 F. is necessary 

 for the full development of the parasite n the fly, ending in 

 invasion of the salivary glands. For two days after the trypa- 

 nosomes have been swallowed by the fly they remain infective if 

 injected into a vertebrate, but after this time they must pass 

 through the crithidial stage before they are again infective. 



The reader will note that 

 no sexual reproduction, 

 such as is so conspicuous in 

 the mosquito cycle of the 

 malarial parasites, has been 

 described in this fly cycle 

 of the trypanosome, though 

 the general features of the 

 cycle are so parallel. It 

 can hardly be doubted that 

 sexual reproduction of some 

 kind, or at least something 

 which takes the place of it, 

 does occur in the tsetse fly, 

 but it has ' not yet been 



recognized by Scientific Ob- FIG. 22. Method of division in trypano- 



somes. A, elongated form ready for division; 

 B, form with divided parabasal body and par- 

 Life Cycle in Man. The tially split undulating membrane; C, form with 

 j. i i double parabasal body, double undulating 



parasites, when injected membran P et and double nucleus; D, almost 



into man Or Other SUSCepti- completely divided forms, adhering by poste- 



ble animals by a tsetse fly, 



live and multiply in the blood, swimming free in the serum with- 

 out entering the corpuscles (Fig. 21 A). They obtain nourishment 

 by simply absorbing food material through the delicate cuticle which 

 covers them. The method of division is the usual protozoan type of 

 simple fission. When about to divide the trypanosome elongates 

 (Fig. 22A) and the parabasal body at the posterior end divides 

 first (Fig. 22B). Then the flagellum and undulating membrane 

 begin to split from the posterior end forward, the central nucleus 

 divides (Fig. 22C), and the animal splits into two parts which hang 

 together longest by the " snouts" or posterior ends (Fig. 22D). 



