TRYPANOSOMES IN TSETSE-FLIES AND OTHER DIPTERA. 175 
neighbourhood of Entebbe; I shall refer to this monkey 
briefly as the fresh-fly monkey. I have also preparations of 
cerebro-spinal fluid from patients, and of the blood of a 
chimpanzee, eighteen days after inoculation with human 
cerebro-spinal fluid. I have no preparations of human blood 
showing trypanosomes. 
Inall cases alike the trypanosomes are distinguishable by 
their general build and appearance into slender, stout, and 
intermediate forms.!. In the living condition the slender 
forms appear more snake-like, and are active in their move- 
ments, while the stout forms are fish-like in shape, and less 
motile. In successful preparations of blood-smears the three 
forms can usually be distinguished readily, especially in films 
fixed with osmic acid vapour, which preserves the body-form 
very perfectly. In films preserved by the ordinary drying 
method the different forms appear less differentiated. This 
is evidently the result of a slight flattening-out of the body, 
produced by the method of drying. ‘The two extremes of 
the series—namely, the stout and slender types—contrast 
with each other, not only in their proportions, but also in the 
relative length of the free flagellum, which is long in the 
slender forms, short in the stout ones. ‘The trypanosomes of 
the intermediate type can also be subdivided into two types 
by this character, so that all the trypanosomes in a given 
blood-preparation can be divided into two classes—those with 
long and those with short flagellum. ‘This character, which 
is plainly seen in the figures of Bruce and Nabarro (5), is 
more reliable for use as a morphological distinction than the 
stoutness of the body—a character lable to alteration or 
deformation due to imperfect preservation. 
‘he extreme of the stout type is seen in the so-called 
1 Moore and Breinl (80) state that they are unable to distinguish any marked 
dimorphism, and that the so-called males and females are “arbitrarily chosen ” 
extremes in a continuous series. It was never pretended that they were any- 
thing else than the extreme differentiations, obvious naturally, of a neutral or 
intermediate type; as will be described below, after twenty-four hours in the 
invertebrate host the intermediate forms disappear, and only the extremes 
remain, arbitrarily selected or differentiated by the action of their environment. 
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