STUDIES ON AVIAN H.EMOPROTOZOA. 691 



by flagellate forms known to occur in various blood-sucking- 

 invertebrate hosts. 



I may now conti'ast with them certain other cultural forms 

 found, most of which I have equally little hesitation in con- 

 sidering as abnormal or atypical forms, developed by the 

 parasites as a result of unfavourable conditions in the medium. 

 These forms are found in old, original cultures of, say, 

 twelve days or more, in which multiplication has gone on to a 

 very great extent. It must be boriie in mind that sucli a 

 medium no longer corresponds at all to any condition met 

 with in an insectan host. In an insect, the digestion of the 

 imbibed blood — the medium of the parasites — and its absorp- 

 tion ;ire completed in the course of a few days at most; by 

 this time the parasites remaining in tlie digestive tract have 

 passed into the resting, attached phase. In an old culture, 

 on the other hand, the fluid medium is still present, presumably 

 containing a certain amount of nutriment of a kind, but now 

 considerably altered in character by the addition of waste 

 products of the metabolism of the parasites, which have 

 doubtless a deleterious action on the trypanosomes. In sub- 

 cultures made at sufficiently short intervals, these abnormal 

 forms are usually not found at all. In this case it is as if the 

 transferred parasites remained continuously in a pure medium, 

 which may be looked upon as a substitute for the medium 

 in the stomach of the insect — at any rate during the early 

 period of digestion. 



A most interesting feature of the morphology of these 

 forms is that very few of them show the trypanomonad phase; 

 neai'ly all the parasites have passed into a more or less 

 herpetomonad-like condition. The earliest indication of an 

 alteration in the character of a culture is afforded by the 

 appearance of such forms. They are to be met with in 

 cultures of ten or twelve days and onwards. At first, of 

 course, these individuals are very few in number. 



Examples of this " pseudo-herpetonionad " condition, as I 

 propose to term it, are seen in figs. 140-146 ; figs. 140, 145, 

 and 146 are from a chaffinch culture of twelve days; fig. 141 



VOL. 55, PART 4. NEW SERIES. 46 



