448 



FAMILY: TRYPANOSOMID^E 



T. congolense. The parabasal part of the kinetoplast varies considerably. 

 It is a comparatively large body, often slightly elongated, or egg-shaped in 

 T. cruzi. It is smaller and spherical in most pathogenic forms, while in 

 T. equinutyi of the South American disease of horses (mal de Caderas), it 

 is apparently absent (Plate V., i, p. 456). 



In a single blood-film from an infected animal it will be found that the 

 trypanosomes are not all of the same size, and in the case of some patho- 

 genic forms {T. gambiense, T. brucei) it has been the custom to describe 

 those present as belonging either to the " short stumpy," " intermediate," 

 or " long thin " forms (Figs. 221, 224). The measurement of the try- 

 panosomes and the relative positions of the various structures may be 

 roughly made by the micrometer eye-piece, but the flagellates are often 



T.evansi 



T. brucei 



Fig. 196. — Curves representing the Distribution, by Percentages, in 

 Respect to Length of Trypanosoma evansi and Trypanosoma brucei. (After 

 Bruce, 1911.) 



so twisted in the stained film that this is a matter of difficulty. A more 

 accurate method is to draw the trypanosomes to a fairly large scale with 

 the aid of a camera lucida, and to project on to the paper the micrometer 

 scale through the same system of lenses. The trypanosomes may then 

 be measured by setting the measuring dividers according to the scale on 

 the paper. A large number of individuals can thus be measured, and the 

 average dimensions taken, while a curve can be plotted showing the 

 percentage of trypanosomes of any length between the maximum and 

 minimum (Fig. 196). A feature of these curves is that very frequently 

 they show a notch which has been interpreted as an indication that in such 

 infections two particular types of trypanosomes are present — not neces- 

 sarily two species, but that the one species tends to produce in its develop- 



