534 



Miss M. Robertson. Polymorphism of 



[July 5, 



a glance at the tables will show, a relative decrease in the numbers of the 

 long and intermediate individuals as soon as there is a lull in the number of 

 the divisions. At the height the divisions cease or drop to a very low 

 percentage, and correlated with this there is a reduction in .the two types 

 just mentioned. This disappearance of the long and intermediate forms is 

 so marked that, just before the large drop in absolute numbers sets in, there 

 is a state of affairs in which once more the shorter range of forms is almost 

 the only one represented (see Table 16). 



It is rather important to note that the main drop in numbers is usually 

 preceded by a slight but quite definite numerical reduction which seems to 

 set in after the divisions begin to slacken, so that it appears as though the 

 inhibition of the divisions were already accompanied by a certain amount of 

 actual destruction of the trypanosomes, though this is not yet serious in 

 extent. This check in the divisions before the large drop in the numbers 

 is evidence in favour of the serum exerting a harmful influence upon the 

 parasites. 



The inhibition of division can hardly be an effect of direct phagocytosis, 

 and suggests that a certain amount of trypanolysis may occur in the 

 circulating blood-stream. The vacuolated appearance of the trypanosomes 

 to be noted in some cases just before or during the drop seems to lend 

 further weight to this view. 



Now one of the most striking points in the foregoing account is that the 

 forms about to divide are the long slender ones. Figs. 4 and 5 give the 

 typical picture of the earliest stages. The disappearance of this form when 

 the divisions cease is also most striking in this connection, so that one is led 

 to the conclusion that the long types are forms in preparation for division. 



Briefly therefore my interpretation of the endogenous cycle is as follows : — 

 The short forms (15-20 yu,) constitute the normal adult blood type ; this 

 expression is merely used to indicate the form which has the largest duration 

 in time in the cycle and which is the most stable. These increase in size 

 and bulk, and form that sliding scale of individuals which may be termed the 

 intermediate forms ; these in turn lengthen out to the long, more slender 

 types, which proceed to divide, giving rise once more to the short forms. The 

 products of division are often unequal to a varying extent. One individual 

 is usually much shorter, has a very short free flagellum or none at all, while 

 the other partner may be of considerable length. Where this occurs there 

 is a suggestion that the longer individual may proceed to divide again within 

 a very short period, but the point is not very clear. 



Early dividing individuals are found more nearly approximating to the 

 intermediate type, but they are not numerous (figs. 12 and 13) and appear to 



