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developed  from  pathogenic  ones.  If  they  are  derived  from  these,  and 
not  from  the  ordinary  intestinal  forms,  there  does  not  seem  to  be 
any  reason  why  the  development  in  the  proboscis  should  only  occur 
in  10  per  cent,  of  the  flies  fed,  and  not  in  all,  and  this  is  the  percentage 
in  which  intestinal  infection  was  present  in  his  flies.  Moreover,  he 
does  not  give  any  reason  for  stating  that  the  intestinal  parasites  are 
cultural  forms  of  T.  fecaudi  which  must  have  been  derived  from  big 
game.  In  the  first  place,  it  is  extremely  rare  to  find  trypanosomes  in 
buck,  and,  secondly,  it  does  not  follow  that  such  parasites  as  may  be 
present  are  T.  pecaudi.  We  do  not  wish  to  criticise  unnecessarily, 
but  in  working  with  tsetse  flies  great  caution  must  be  observed  before 
reaching  definite  conclusions.  The  greatest  objection  to  the 
assumption  that  the  forms  seen  in  the  proboscis  are  derived  from 
pathogenic  trypanosomes  is  that  they  are  innocuous  w'hen  injected 
into  susceptible  animals. 
Our  results  are  discordant,  for  in  Gl.  morsiians,  both  intestinal  and 
proboscidal  forms  were  encountered,  while  in  Gl.  palpalis  only  the 
first  were  seen.  This  may  be  accidental,  and  more  continued  work 
might  have  resulted  positively. 
Of  the  two  hypotheses  we  are  inclined  to  agree  with  that  of 
Stuhlmann  :  that  the  parasites  in  the  proboscis  are  derived  from  those 
in  the  intestine. 
It  is,  perhaps,  possible  that  the  parasites  occurring  in  ‘  wild  ’  tsetse 
flies,  as  distinct  from  laboratory  bred  ones,  may  be  derived  from 
pathogenic  trypanosomes  which  lose  for  some  unknown  reason  their 
infectivity  when  ingested,  and  it  is  possible  that  the  intestinal  forms 
do  not  all  belong  to  one  species  but  represent  a  mixed  infection. 
This  can  only  be  decided  by  lengthy  experiments. 
In  the  case  of  Gl.  morsitans  it  would  seem  on  a  priori  grounds 
that  a  development  of  the  pathogenic  trypanosomes  does  occur,  for 
it  is  a  reasonable  certainty  that  any  susceptible  animal  exposed  to 
their  bites  in  nature  will  become  infected.  All  such  infections  cannot 
be  explained  by  a  mechanical  transmission.  The  recent  work  of 
Kleine,  and  its  confirmation  by  Bruce,  shows  that  a  development 
apparently  does  occur. 
