412 Col. Sir D. Bruce and others. The Development of [July 5, 



The mid-gut. — The mid-gut contained innumerable trypanosomes of the 

 gambiense type. Some are dividing, and all have a well-marked nucleus and 

 micronucleus, the latter at or near the posterior extremity. The protoplasm 

 contains many chromatin granules, and an undulating membrane and 

 flagellum are present '(Plate 10, figs. 6 — 16). Many groups, or rosettes, 

 composed of 15 to 20 individuals, occur, the flagella pointing outwards 

 (Plate 11, fig. 1). 



The proboscis, proventricidus, thoracic gut, crop, hind-gut, and Malpighian tubes 

 contained no trypanosomes. 



The most interesting thing in this description of the examination of 

 Ply 866 is the condition of the salivary glands. How these trypanosome- 

 like bodies, or derivatives of trypanosomes, got into them is a mystery, and 

 we will content ourselves at present with merely placing the bare fact on 

 record until the salivary glands of similarly infected flies are examined. 



There is one fallacy which might be pointed out. It is assumed that 

 Fly 866 became infected on the first or second day of the experiment. It is 

 possible that it became infected when feeding on the fifth clay on an animal 

 which showed trypanosomes in its blood a day or two later. This, however, 

 is unlikely, as no other fly showed trypanosomes on dissection. 



In order to make the story more complete, on Plate 10, figs. 1 — 5, is repre- 

 sented the Trypanosoma gambiense from the blood of one of the monkeys on 

 which the flies were fed at the beginning of the experiment, and on Plate 11, 

 figs. 2 — 5, are shown Trypanosoma gambiense from the monkey which became 

 infected from the contents of the mid-gut of Ply 866. 



Proportion of Infected Flies to Non-Infected in Nature. 



In the experiment under consideration it is seen that, in artificially- 

 infected flies, only 1 in 60 showed the phenomenon of late infectivity. In 

 nature the proportion must be less, as many of the flies, in many places at 

 least, can never have fed on an animal whose blood contained Trypanosoma 

 gambiense. 



That there can be but few under natural conditions Table IV shows. The 

 table is made by subtracting the flies fed on the animal during the last 

 seven days, before trypanosomes were found in the blood, this being the 

 incubation period, from the total number. The experiments consist in 

 catching tsetse flies in the infected area, bringing them to the laboratory and 

 placing them straightway on healthy animals. 



The first two experiments were made with Trypanosoma brucei and 

 Glossina morsitans, and it would appear from them that 104 and 108 flies 



