Carrier of Trypanosome Disease in Nyasaland. 27 



glands and contents of the intestine also failed to infect Iiat 2234. It would 

 appear, then, that this fly had not yet reached the infective stage. 



It is now proposed to describe the different forms of the trypanosomes 

 found in the salivary glands of this fly somewhat in detail, and to bring 

 forward a theory in regard to a stage which seems to occur in the final 

 development of this trypanosome in the salivary glands. The different 

 forms and apparent stages in their development could be more easily made 

 out in G. hrevifalfis than in G. morsitans, on account of its greater size. 

 On Plate 1 this evolution of the trypanosome causing disease in man in 

 Nyasaland in the salivary glands is represented. 



Figs. 1 and 2 represent the long, slender developmental forms of trypano- 

 somes found in the intestine of the fly, from the mid-gut to the proventriculus. 

 It is this type of trypanosome which invades the salivary glands. 



Fig. 3 shows the change in shape which the intestinal forms undergo on 

 entering the salivary glands. The posterior extremity lengthens somewhat 

 and the micronucleus and flagellum pass forward. This appears to be the 

 commencement of the change to the crithidial type. 



Fig. 4 represents the fully developed crithidial form of the parasite. The 

 micronucleus and flagellum have passed further forward until they lie 

 anterior to the nucleus. The anterior portion of the parasite has broadened 

 out ; the posterior has become attenuated. 



Fig. 5, the parasite is still crithidial in form. The anterior half has become 

 still broader, the posterior half elongated and further attenuated. 



Fig. 6, the attenuated posterior portion has shortened. 



Figs. 7, 8 and 9 represent further stages in the evolution. The long 

 attenuated posterior extremity has disappeared and the parasite has become 

 contracted and thickened. 



Fig. 10 shows the last phase of the crithidial stage. Here the parasite has 

 assumed a rounded form and the flagellum is folding on itself. 



Fig. 11 represents a group of parasites in the encysted stage. In this form 

 they are found massed together in the lumen of the salivary glands. 



Fig. 12, in this group the encysted forms are just unfolding. 



Fig. 13 shows a single encysted form breaking open. The micronucleus is 

 now posterior to the nucleus ; the crithidial has become the trypanosomal. 



Figs. 14, 15, 16, and 17 demonstrate further stages in the unfolding of the 

 encysted form. The parasite is now assuming a trypanosome shape. 



Figs. 18 and 19 show the fully developed salivary-gland form of the 

 trypanosome. This constitutes a reversion to the " blood form " from which 

 the cycle of development began and is the only infective form. 



On comparing these figures with the developmental forms of the 



