530 Miss M. Robertson. Polymorphism of [July 5, 



in the earlier periods of the infection when the absolute numbers are 

 relatively high. I have worked exclusively with monkeys, but no doubt 

 the process is very similar in other animals infected with T. gambiense. 

 Superficially, guinea-pigs and rats seem to behave as monkeys in regard to 

 the fluctuations, but no detailed study has been made of them. 



The coming and going of the trypanosomes described above may be con- 

 sidered to be the endogenous cycle in the blood of the vertebrate. 



To consider now the polymorphism of the trypanosome and its relation (1) 

 to the endogenous cycle, (2) to the production of infected flies, i.e. to the 

 exogenous cycle in the fly. 



III. Method. 



The method employed in studying the trypanosome is as follows. Blood- 

 films are taken daily from a gambiense monkey at the same hour (9 a.m.) ; 

 are fixed by exposure to osmic vapour for half a minute ; are immediately 

 plunged into absolute alcohol and left for 10 to 20 minutes. Then they are 

 dried in air and stained with Giemsa's fluid. 



This method, while it gives the worst possible picture of the nuclear detail, 

 has nevertheless excellent qualities in regard to the type of result at present 

 required. To begin with, it fixes all the trypanosomes present in the drop of 

 blood, which no true " wet method " does with absolute certainty ; it flattens 

 the creatures into one plane so that they may be more easily drawn, it is 

 rapid, the errors are apparently very uniform, and it agrees with the method 

 employed by other workers. 



Whenever possible 100 trypanosomes were drawn with the camera at 2000 

 magnification. They were measured by means of a compass set at a distance 

 corresponding to one micron, as in Bruce's method. The results were 

 tabulated, and finally plotted out on squared paper and a curve drawn. In 

 cases where the trypanosomes were very scarce 25 or 20, and in one instance 10, 

 trypanosomes had to suffice ; it is, however, obvious that in these cases the 

 smaller numbers are quite as good a sample of the total forms present as 100 

 individuals on days when the parasites were more numerous. 



The above method of measuring has been criticised by Dr. F. W.W. Stephens,* 

 who recommends the projection of the trypanosomes on to a screen in a dark 

 room and considers that the actual measurement should be done by the 

 " tangent line method." 



Drawing by means of the camera is obviously open to criticism, but the 

 projection of the trypanosomes on to a screen is a counsel of perfection 

 beyond the present achievement of workers in Central Africa. Dr. Stephens' 

 * ' Sleeping Sickness Bulletin 36,' 1912, No. 4. 



