44 ZOOLOGY FOR MEDICAL STUDENTS chap, i 



in number, it is found that the trypanosomes which survive through 

 this " period of depression " are characterized by their short stumpy form 

 (Fig. 19, A). These short stumpy trypanosomes, which are apparently 

 endowed with special powers of resisting unfavourable conditions, are of 

 peculiar practical importance for when a meal of infected blood is taken 

 in by the transmitting insect — ^the blood-sucking fly Glossina palpalis — 

 they alone survive and serve to infect the fly, all the other trypanosomes 

 being killed and digested. It follows that the blood of a particular host 

 is infective only when trypanosomes of this short stumpy type are 

 present. 



When blood containing them is swallowed by the Glossina the trypano- 

 somes multiply actively in the fly's alimentary canal until they fill the 

 whole intestine as a seething mass. Much variation is seen in both size 

 and shape (Fig. 19, B, C, D) but at a period varying from about the eighth 

 to the eighteenth day there begin to make their appearance trypanosomes 

 which are conspicuous by their peculiarly long and slender form (Fig. 19, 

 E). These gradually work their way forwards in the cavity of the aliment- 

 ary canal and eventually (sixteenth to thirtieth day) make their way into 

 the salivary glands (see p. 231). Here they attach themselves by their 

 flagella to the lining of the gland and sway about within the cavity. 

 They multiply actively, they become shorter in fojrm and many are found 

 to have the kinetonucleus on the flagellar side of the trophonucleus 

 (Fig. 19, G). This is known as the Crithidial type from the name of a 

 genus Crithidia which is distinguished from Trypanosoma by the position 

 of its kinetonucleus. Amongst the others there also appear stumpy 

 trypanosomes (Fig. 19, H) resembling those of the blood and it is 

 apparently these which, injected into the blood when the Glossina bites, 

 serve to start a new infection in the mammalian host. Ordinarily these 

 infective trypanosomes make their appearance in the salivary glands 

 from about the twentieth day after the fly has taken in the infected 

 blood. 



It will be noticed from the above life-history that the Glossina which 

 serves to carry the trypanosome infection from one mammalian host 

 to another is rendered capable of successfully inoculating the latter 

 by the presence in its salivary glands of the short stumpy type of trypano- 

 some which marks the final stage of the cycle within the fly. Here we 

 have to do with what is known as the cyclical type of infection — dependent 

 upon the completion of a definite life-cycle within the intermediate 

 host. This contrasts strongly with what is known as direct infection 

 in which a microbe is conveyed directly from one host to another 

 by simple transference. Infection would be direct were individual 



