290 THE PROTOZOA 



whether it be the blood-sucking invertebrate, or possibly the flesh 

 or organs of another vertebrate infected with trypaiiosomes. 



Two methods of inoculative transmission of trypanosomes have 

 been distinguished ; in the one, known as the " direct " or " mechan- 

 ical" method, the parasites merely become contained in or adhere to 

 the proboscis of the blood-sucking intermediary when it sucks blood 

 from an infected animal ; and when it feeds a second time the try- 

 panosomes pass directly, and without having undergone any change 

 or development, into the second host ; in the other, known as the 

 " indirect " or " cyclical " method, the trypanosomes, when taken up 

 by the blood-sucking invertebrate, go through a developmental cycle 

 in it, at the end of which, but not before, they are " ripe " for inocu- 

 lation into a suitable vertebrate host. Comparing natural with 

 artificial processes of infection, in the direct method the blood- 

 sucking invertebrate may be said to play the role merely of an 

 injection-syringe, but in the indirect method it acts also as a culture- 

 medium, in which the parasite passes through various phases and 

 assumes forms quite different from those occurring in vertebrate 

 blood. Patton (393) has put forward the view that transmission 

 is always by the. direct method, and that the crithidial and other 

 forms found in the blood-sucking invertebrate are parasites of the 

 invertebrate alone, and have no connection with the trypanosomes 

 found in vertebrates ; but the number of cases in which it has 

 now been shown clearly that trypanosomes go through a definite 

 cycle in the invertebrate host disproves Patton's contention, and 

 renders it unnecessary to discuss it further. It is rather the 

 direct method that stands in need of further demonstration ; though 

 undeniably possible as a laboratory-experiment, it may be doubted 

 if it ever really occurs in Nature, and in any case it is probably to 

 be regarded as a purely accidental rather than a normal occurrence. 



It has been frequently asserted or assumed that trypanosomes 

 can pass from parent to offspring, by so-called " hereditary trans- 

 mission," in the invertebrate host, but convincing proof of this state- 

 ment is as yet lacking entirely. Attempts to prove hereditary trans- 

 mission by direct experiment have given, for the most part, negative 

 results, and the observation so frequently made, that leeches, tsetse- 

 flies, fleas, mosquitoes, etc., bred from the egg and not exposed 

 to infection, are entirely free from parasitic flagellates, affords cumu- 

 lative evidence against the existence of any such method of trans- 

 mission (cf. Kleine and Taute, 459). Brampt (419), however, asserts 

 that T. inopinatum is transmitted hereditarily from parent to off- 

 spring of the leech Heldbdella algira. According to Porter (554), 

 " Criihidia " melophagia of the sheep-ked is also transmitted from 

 parent to offspring in this insect ; and if, as is extremely probable, 

 the flagellate in question is the developmental phase of the trypano- 



