Diseases of Domestic Animals in Nyasaland. 



37 



Figs. 9-11 are the ordinary forms found clinging by their flagellar ends to 

 the labrurn. It will be seen that they have assumed the crithidial stage, a 

 stage which seems to be a sine qud non in the final stages of the cycle of 

 development of all the pathogenic trypanosomes, and the interpretation of 

 which is still obscure. 



Figs. 12-19 are various forms other than " blood forms" which have been 

 squeezed out of the proboscis of a living infective fly. Fig. 15 appears to be 

 encysted. 



Figs. 20-29 are " blood forms " from the hypopharynx of dead infective flies, 

 and also from living flies induced to salivate on a cover-glass. They represent 

 the final stage in the cycle of development and are the only infective forms. 



Conclusions. 



1. That T. pecorum is capable of passing through a cycle of development in 

 G. morsitans, the flies becoming infective some 20 days after feeding on an 

 infected animal. 



2. That T. pecorum belongs to the same group as T. simice, the development 

 taking place at first in the gut and afterwards passing forward into the 

 labial cavity and finally into the hypopharynx. 



3. That the final stage of the development only occurs in the hypopharynx, 

 where the trypanosomes revert to the " blood form " and become capable of 

 setting up infection if injected under the skin of healthy animals. 



DESCRIPTION OF PLATE. 

 (See also pp. 36 and 37 above.) 

 Figs. 1 and 2, trypanosome forms from proventriculus. 



Figs. 3-8, early infection of the labrum ; the flagellates still retain the trypanosome 

 characteristics. 



Figs. 9-11, ordinary crithidial forms found adhering in masses to the labrum. 

 Figs. 12-19, other forms from labial cavity. 



Figs. 20-29 represent the final stage of the development in the hypopharynx — the 

 infective or " blood form." 



Stained Gienisa. x 2000. 



