738 MEDICAL ENTOMOLOGY 



duct in the hypopharynx are in close apposition, they are quite distinct 

 channels, and communication between the two is only established at 

 their openings, if at all. It is clearly necessary, therefore, that the 

 trypanosomes, if they do not penetrate the wall of the gut and make 

 their way to the salivary glands from the outside, must pass up the food 

 channel to the tip of the proboscis and then turn round the corner and 

 make their way back in the reverse direction up the hypopharynx. In 

 other words, they pass through the valvular proventriculus, up the 

 empty and contracted oesophagus, through the pharynx and the con- 

 nected sphincter muscles, which are specially designed to prevent the 

 forward passage of the contents of the gut, and down the narrow- 

 channel between the labrum-epipharynx and the hypopharynx ; they then 

 turn directly backwards, past the valve formed by the patulous tip of 

 the hypopharynx, up the salivary duct and through the salivary valve, 

 designed to permit of only a forward flow of the saliva, and into the 

 glands. Further, during all this time they must necessarily travel in 

 the reverse direction to the normal flow of the contents ; if a feed of 

 blood is taken while they are passing forwards they will have to maintain 

 their position against a relatively enormous mass of blood, quickly 

 absorbed; if they have reached the salivary duct they will have to 

 maintain their position against the flow of saliva. 



It should be noted further that the wall of the alimentary tract in the 

 anterior portion would be much more difficult of penetration than in the 

 lower parts. All the fore-gut is lined by a chitinous intima, and the 

 pharynx and the parts in front of it are relatively very stout. If the 

 trypanosomes pierce the wall at all the most probable situation would be 

 in the mesenteron, where there is no chitinous lining and where the wall 

 is being constantly weakened by the denudation of cells in the process 

 of digestion. The failure to find trypanosomes in the body cavity is, of 

 course, no proof that they were absent at all times. It is reasonable 

 to suppose that if only a few reached the glands they might establish 

 a permanent infection. 



It is not easy to accept the hypothesis that a parasite can pass forward 

 to the proboscis and so into the blood of a vertebrate host at the time 

 of feeding ; it is infinitely more difficult to believe that having reached 

 the proboscis it should again pass backwards, when it might quite well 

 attain its destination by passing directly into the blood. 



The presence of the parasites (not necessarily in stages which would 

 be infective for the vertebrate) in the ovaries leads to hereditary 

 transmission, mariy clear examples of which have been demonstrated. 



