taken  from  the  same  batch  of  normal  rats  which  were  afterwards 
used  for  infection,  but  in  no  instance  have  we  been  able  to  observe 
any  parasites  in  the  gut  contents  resembling  the  developmental 
forms  of  trypanosomes ;  only  in  rare  instances  were  very  small,  round 
cells  observed,  which  resembled  gregarines.  So  far  we  have  not  been 
able  to  come  to  any  definite  conclusion  concerning  the  nature  of  these 
bodies. 
When  lice  in  small  numbers  were  placed  on  a  clean  rat,  they  were 
often  rapidly  eaten  off  by  the  animal  and  disappeared  completely. 
On  the  other  hand,  the  lice  sometimes  multiplied  with  great  rapidity, 
so  that  the  fur  of  the  animal  was  soon  swarming  with  them,  and  the 
animal  itself  soon  succumbed  to  their  injurious  effects.  Prowazek 
has  already  drawn  attention  to  this  difficulty. 
Freak  examination. 
For  fresh  examination,  the  gut  contents  of  lice  were  teased  out 
in  physiological  saline  solution,  and  a  coverslip  preparation  ringed 
witli  hard  paraffin  was  made.  In  these  specimens  the  trypanosomes 
could  be  kept  alive  in  an  actively  motile  condition  for  about  thirty 
hours.  It  is  a  striking  fact  that  if  the  lice  were  obtained  from  rats 
in  an  early  stage  of  infection,  the  trypanosomes  in  the  gut  were  of 
large  size  and  moved  sluggishly.  When  the  rats  were  in  a  later  stage 
of  infection,  the  trypanosomes  were  small  and  moved  at  a  very  rapid 
pace,  shooting  across  the  microscopic  field,  so  that  detailed  observa¬ 
tion  was  almost  impossible. 
To  obviate  this  difficulty  a  slide  was  first  covered  with  a  thin  film  of 
gelatine  in  saline  solution,  by  which  means  the  progressive  movement 
of  the  trypanosomes  was  retarded.  The  gut  contents  of  from  three 
to  four  lice  taken  from  a  rat  at  a  suitable  stage  of  the  infection  were 
then  taken,  and  specimens  were  made  in  the  above  described  manner. 
The  gut  contents  were  seen  to  be  full  of  parasites,  some,  large  and 
moving  sluggishly,  others,  small  and  resembling  in  their  appearance 
living  spermatozoa,  ]30ssessing  a  small  round  head  and  a  long  wavy 
flagellum.  In  some  instances  a  marked  transformation  was  seen  to 
take  place.  A  large  trypanosome  became  altered  in  shape  ;  a  con¬ 
striction  appeared  at  a  point  distant  from  one  end  about  one-third  of 
the  length  of  the  parasite.  This  gradually  became  more  and  more 
pronounced,  at  times  disappearing,  but,  us  a  rule,  in  from  two  to  four 
