E. A. Minchin 
237 
flagellum is also very short, especially the free portion. The ordinary 
form, which is much the commonest on the slide, is more or less 
intermediate between the two extreme forms. There is nothing new 
in these statements, and it seems almost necessary to apologize for 
restating such obvious facts; it is to be hoped that it will not be 
necessary to do so again. 
As regards the significance of these forms, I incline to the opinion 
that the two extremes, slender and stumpy, represent sexual forms 
differentiated in opposite directions from a neutral type. This view 
receives support from the observation made by me, that after about 
12 hours in the tsetse-fly only slender and stout forms are to be found. 
Nevertheless the question of the meaning of this well-marked 
trimorphism cannot be regarded as definitely settled in the present 
state of our knowledge. 
