248 H. M. WOODCOCK. 



at least indicative of tlie general course of events in the 

 usual life of a Trypanosome. For one thing, there can be 

 now little or no doubt that most, if not all, Trypanosomes 

 possess an alternate, Invertebrate host, in which a definite 

 part of the life-cycle, including sexual conjugation, is under- 

 gone. Again, it is very probable that many are capable of 

 entering upon a resting, attached phase, at different periods 

 of the life-history, during which the parasites lose, for the 

 time being, their trypaniform nature, and become gregarini- 

 f orm. In certain cases, indeed, they have gone even farther, 

 and acquired a completely intra-cellular, or H^emosporidian 

 phase.^ 



The facts in support of the above-mentioned propositions 

 are most conveniently treated more or less separately. 

 Apart from the many Invertebrates which are known, or 

 with good reason suspected, to be, at least, " carriers " of 

 Trypanosome parasites (see below in the vSystematic section), 

 there are certain other Trypanosomes, from different Verte- 

 brate hosts, for which the possession of an alternate In- 

 vertebrate host has been, practically speaking, proved. Per- 

 haps the most important instance, since it relates to a 

 Mammalian form, is that lately described by Prowazek (88).2 

 This author finds that T. lewisi undergoes an essential part 

 of its life- cycle in a louse, H^ematopinus. 



Soon after their arrival in the mid-gut the parasites 

 undergo reduction of the nuclear apparatus, preparatory to 

 conjugation. By this means the number of chromosomes is 

 reduced from sixteen to four. The differences between the 

 gametocytes of different sex (male and female Trypanosomes) 

 are not well marked. A noteworthy distinction from the 

 instances above described is that the male form (comparable 

 to a microgametocyte) does not give rise to several micro- 



' The derivation of the Hajmatozoa as a whole, and the direction in 

 which evolution appears to have tended, are considered in Section XI. 



- As the writer has only had access to Prowazek's work since tliis article 

 was sent to tlie publisliers, it is impossible to do more than summarise the 

 principal features. 



