4i6 
not,  they  were  unable  to  decide  definitely.  They  confirm  the  above- 
mentioned  observations  of  Breinl  and  Nierenstein,  laying  particular 
stress  on  the  point  that  a  slight  resistance  may  still  be  noticed  when 
Atoxyl-resistant  trypanosomes  are  subinoculated  into  a  different 
species,  such  as  from  mice  into  a  dog  or  a  guinea-pig. 
Rbhl*'  states  that  trypanosomes  which  are  Atoxyl-resistant  in 
mice  are,  on  inoculation  into  rats,  not  absolutely  resistant,  though 
certainly  so  to  some  extent.  He,  however,  does  not  explain  whether 
this  resistance  was  only  noticeable  in  the  first  generation.  Further¬ 
more,  he  brings  forward  experimental  evidence  which  tends  to  show 
that  an  Atoxyl-resistant  strain  in  mice  on  subinoculation  into  rats  is 
not  affected  by  Atoxyl.  However,  as  inoculation  and  injection  were 
done  simultaneously,  in  our  opinion  no  definite  conclusion  can  be 
formed. 
We  regard  the  resistance  as  an  acquired  immunity  of  the  trypano¬ 
somes  against  the  Atoxyl-serum,  as  we  have  previously  pointed  out. 
This  is  in  accordance  with  our  observation,  confirmed  by  different 
observers,  that  the  Atoxyl  resistance  only  holds  good  for  the  one 
species  m  which  it  has  been  acquired.  The  trypanosomes  have 
become  tolerant  only  to  the  one  Atoxyl-serum  combination,  as  for 
example  in  the  mouse,  and  are  still  influenced  by  the  Atoxyl-serum  of 
the  rat. 
y\  similar  view  has  been  expressed  by  Mesnil  and  Brimont.f  In 
further  experiments,  however,  with  an  Atoxyl-resistant  strain  in  mice, 
sent  to  us  by  Professor  Ehrlich,  we  observed  that  the  resistance  after 
subinoculation  from  mice  into  rats  was  still  well  marked  in  the  latter 
animals  even  after  three  passages.  This  result,  also  obtained  by 
Rbhl,  seemingly  contradicts  our  previous  observations  that  the 
lesistance  is  confined  to  one  animal  species  only.  We  agree  with 
Rohl  s  opinion  that  the  seemingly  contradictory  results  concerning' 
resistance  can  only  be  explained  through  the  assumption  of  a  gradual 
increase  of  the  resistance,  after  numerous  passages  from  a  single 
animal,  at  first  to  animals  of  the  same  species,  and  later  to  animals  of 
a  different  species. 
K-ohl.  Rerl.  Klin.  Wochenschrift,  No.  ii,  1909. 
t  Mesnil  et  Brimont.  Annales  de  I’Instit.  Pasteur,  Tome  XXII,  p.  856,  1908. 
