1912.] T. gambiense and its Relation to G. palpalis. 533 



They have a short free flagellum, or may show practically no free flagellum, 

 though it is never easy to he perfectly certain of the exact point where the 

 body may be said to end absolutely and flagellum alone to exist. The 

 breadth varies, but the forms are usually rather broad, measuring about 

 2 to 2'50 microns. In the course of the next few days there may be a 

 level depressed period, with little or no increase in number, and there is 

 correspondingly little change in type. As already said, "the depressed period 

 may be very short, or may last a number of days. The time factor, it must 

 be emphasised, is open to the most capricious variation at all periods of the 

 cycle. At the time when the first signs of a rise in numbers have occurred, 

 it is found that with these there appears an immediate alteration in type, 

 and long, slender, and intermediate forms are to be found, in addition to the 

 short forms already present. 



The sequence of events can be seen very clearly in cases where the 

 depressed period shows a fair number of trypanosomes and where the rise is 

 relatively slow ; this is a quite frequent occurrence and the conditions are as 

 follows : — 



The first sign that a rise is about to take place is an increase in the size of 

 a number of the trypanosomes ; they become longer and there is a general 

 increase in the length of the free flagellum. The width does not alter at 

 first, and there is thus produced a state of affairs showing only short and 

 intermediate forms, or if the increase in size is very general, intermediate 

 forms may exist for the time being almost alone. This is an important 

 moment in the cycle, as will be seen hereafter ; some deeper physiological 

 change in the trypanosomes accompanies the increase in size, or, rather, sets 

 in at some period during the increase. 



It is to be noted here that the rise in the numbers of intermediate forms 

 (figs. 9-11) at this stage occurs at the expense numerically of the short 

 forms, and one is, it appears, in every way justified in assuming that they 

 are derived one from the other. There now occurs a marked drawing out of 

 the body in a proportion of the parasites, and there are thus produced long, 

 slender individuals such as that shown in figs. 1-3, and immediately upon 

 this, the first burst of division occurs. 



Naturally, if this process just sketched takes place very rapidly or very 

 gradually, or with very low numbers, it is somewhat obscure, as the con- 

 secutive stages do not stand out with sufficient clearness. 



As soon as the division actually takes place there is a reappearance or an 

 increase, as the case may be, of the short forms. So that there are present 

 long, short, and intermediate forms and dividing individuals. As the rise 

 proceeds the relative numbers of these types vary, but there is always, as 

 vol. lxxxv. — B. 2 p 



