THE STRUCTURE OF TRYPANOSOMA LEWISI. 787 
may, therefore, for purposes of description, divide the body 
into three regions: the pre-nuclear region in front of the 
trophonucleus; the inter-nuclear region between the two 
nuclei; and the post-nuclear region behind the kineto- 
nucleus. 
In the inter-nuclear region the body is roughly cylindrical, 
but slightly thicker midway between the nuclei. In the pre- 
nuclear region the body tapers very rapidly to a fine fila- 
mentous prolongation, often difficult to distinguish from the 
flagellum. In the post-nuclear region the body also tapers 
evenly and rapidly to a point. 
The flagellum arises from the blepharoplast or basal 
granule situated in front of the kinetonucleus, and runs at 
once slantingly forwards up to the surface of the body to 
form the marginal flagellum, running along the edge of the 
undulating membrane and continued beyond the anterior 
extremity as the free flagellum. The breadth of the undu- 
lating membrane appears greater or less, according to the 
view presented by the trypanosome; so far as can be judged, 
it is rather more than half the greatest thickness of the body. 
In this trypanosome the undulating membrane is not much 
pleated and runs in even, sinuous curves, corresponding to 
the twists of the body. In any preparation the trypano- 
somes are found in all sorts of positions, but, speaking 
generally, the body usually shows one or two main curves, 
and may be briefly described as either more or less C-shaped 
or S-shaped. In either case the undulating membrane keeps 
always to the convexity of the curves, crossing over the body 
between each bend. ‘lhe explanation of this is to be found 
in the fact that the marginal flagellum is considerably longer 
than that portion of the body along which its runs, as can be 
determined easily by actual measurement; it is shown in the 
table of measurements given below that the average length 
of the pre-nuclear and inter-nuclear regions together 1s 
19°26 n, while the average length of the marginal flagellum is 
23°51. If we imagine the marginal flagellum as an elastic 
fibril joined by the undulatine membrane to a considerably 
