TRYPANOSOMES IN TSETSE-FLIES AND OTHER DIPTERA. 173 
free end of the flagellum. It is stated that the flagellum and 
undulating membrane are formed from the achromatic 
apparatus of a nucleus spindle, of which the central spindle 
gives rise to the flagellum and the mantle fibres to the 
myoneme-fibres of the undulating membrane, while the two 
centrosomes become the blepharoplast. These statements 
have received some confirmation from the observations of 
Robertson (87) on the formation of the flagellum in the 
trypanosome of Pontobdella muricata. 
I have frequently noticed, especially in the slender forms 
of T. grayi, that the flagellum is distinctly thickened at the 
free extremity, but I should not like to affirm the existence 
of a definite blepharoplast at this point. Prowazek (36) has 
also described for T’. lewisi a complicated arrangement 
of anchoring granules and fibrils. I can only say that in my 
material I have not seen them. _ 
With regard to the use of the terms anterior and posterior 
much has been written, and it has been pointed out by 
Woodcock (42) and Liihe (26) that what is morphologically 
anterior in one trypanosome may be posterior in another. It 
cannot, I think, be disputed that there may be two entirely 
different lines of phylogenetic evolution amongst the organisms 
grouped generally as trypanosomes (see below, p. 219), but 
in the present state of knowledge it is not possible to state 
definitely which of the two possible modes of orientation is 
applicable to a given species. Hence it would be best, 
perhaps, to avoid altogether the use of the terms anterior 
and posterior in describing trypanosomes, and to speak of 
the flagellar and anti-flagellar extremities, but such a 
terminology becomes very cumbrous in practice. I shall 
speak of the flagellar extremity as anterior, the anti-flagellar 
as posterior, using the terms in a purely descriptive sense 
and without prejudice to the morphological questions involved. 
So far as I have observed, when trypanosomes are moving 
freely they travel usually with the flagellar extremity directed 
forwards, but when pushing their way amongst blood- 
corpuscles they do so with the free flagellum of the undulating 
