1922] on Microscopic Parasites and their Carriers 



511 



by a single fly during one feed. It is probable that when once 

 infected the fly remains so for the rest of its life. 



In the case, therefore, of both the malarial parasites and those of 

 sleeping sickness and allied diseases, the habit of the insects of inject- 

 ing saliva when they feed has been utilised by the parasites in order 

 to reach the necessary host and to maintain its species. 



Fig. 4. — Trypanosoma gambiense, the Trypanosome of Sleeping Sickness 

 op Man, and its development in the tse-tse ply (Glossina palpalis). 



a-d Trypanosomes in the blood of man. (Magnified 1500 times.) 



a, typical trypanosomes ; d, fully grown dividing forms. 



i, p, s Diagram of development in the tse-tse fly. 

 i, trypanosomes in the stomach ; s, trypanosomes which later enter the 

 salivary glands ; p, proboscis which inflicts the wound and from which 

 the trypanosomes of the salivary glands are injected into man. 



b, c, e Development in the tse-tse fly which terminates in the infective 



forms c, which are the ones which infect man, and are produced in the 

 salivary glands from crithidial forms b, which result from trypanosome 

 forms e which have made their way from the intestine to the salivary 

 glands. (Magnified 1500 times.) 



Vol. XXIII. (No. 116) 2 o 



