630 
ME.  J.  LISTEE  ON  THE  CHTANEOIJS 
the  number  of  divisions  traversed,  these  having  a known  proportion  to  the  depth 
measured.  Opportunities  for  testing  the  correctness  of  this  measurement  were  pre- 
sented by  other  cells  which  lay  edgevdse,  so  that  their  thickness  could  be  observed 
directly. 
Perhaps  the  strongest  argument  in  favour  of  the  cellular  nature  of  these  receptacles 
of  colouring  matter  is  afforded  by  the  universal  presence  of  a nucleus  in  the  central 
cavity  of  each.  In  large  frogs  it  is  often  difficult  or  impossible  to  discover  clear  evidence 
of  it,  but  in  small  ones,  in  which  the  web  is  much  thinner  and  its  constituent  parts 
therefore  capable  of  clearer  definition  with  the  microscope,  it  can  be  quite  distinctly 
seen  in  the  reticular  condition  of  the  pigment.  Its  form  and  relations  may  be  gathered 
from  figs  8,  9 and  10.  In  8 and  10  the  bodies  of  the  cells  are  viewed  on  the  fiat,  and 
the  nucleus  appears  as  an  oval  colourless  body,  about  25^00^^  of  an  inch  long  by  gg^opth 
of  an  inch  broad.  In  fig.  9 the  body  of  the  cell  is  seen  edgewise  applied  to  the  wall  of 
a capillary  blood-vessel,  which  is  embraced  by  its  processes.  The  thickness  of  the 
nucleus  is  thus  displayed,  and  is  shown  to  be  equal  to  that  of  the  cell  in  which  it  lies, 
which  in  fact  it  causes  to  bulge  slightly,  and  also  nearly  as  great  as  the  breadth  of  the 
nucleus  in  figs.  8 and  10.  In  the  cell  of  fig.  10,  the  thickness  of  the  nucleus,  measured 
in  the  manner  above  described,  was  found  about  equal  to  its  breadth.  The  nucleus  in 
fig.  8 is  not  centrally  placed  in  the  body  of  the  cell,  and  I have  in  some  other  cases 
seen  it  still  more  excentric*. 
The  contents  of  these  cells  are  very  minute  dark  granules  or  molecules  suspended  in 
a colourless  fluid,  in  which  I have  often  seen  them  moving  freely : when  in  considerable 
mass  they  produce  a jet-black  appearance,  but  exhibit  a bro^n  tint  when  present  only 
in  small  quantity. 
When  the  skin  of  the  animal  is  very  pale,  the  colouring  matter  is  all  accumulated  in 
the  central  parts  of  the  cells.  With  regard  to  the  method  in  which  this  change  is 
effected,  I am  compelled  to  differ  altogether  from  the  before-mentioned  authorities,  who 
suppose  that  the  granules  and  fluid  are  together  forced  by  contraction  from  the  pro- 
cesses into  the  bodies  of  the  cells.  They  seem  to  take  it  for  granted  that  the  depth  of 
tint  of  any  one  part  of  a cell  depends  simply  upon  the  bulk  of  the  contents  situated 
there,  and  the  consequent  thickness  of  the  coloured  medium  through  which  the  light 
passes  before  reaching  the  eye.  This,  however,  is  by  no  means  the  case,  as  may  be 
seen  by  referring  again  to  Plate  XLVII.  fig.  3.  The  pigment  is  there  represented  fuDy 
diffused  through  the  ramifications  of  the  offsets,  and  some  of  the  smallest  of  these  are 
darker  than  the  bodies  of  the  cells,  and  the  adjoining  broad  parts  of  the  processes ; yet 
the  former  are  far  from  being  thicker  than  the  latter : on  the  contrary,  some  of  the 
branches,  though  conspicuous  for  their  blackness,  appeal’  but  as  delicate  lines  which  can 
be  seen  only  at  one  focus  when  a glass  of  very  high  power  is  employed ; while  the  bodies 
of  the  cells,  as  above  mentioned,  possess  considerable  thickness,  and  the  processes  are 
* The  precise  relations  and  dimensions  of  the  nucleus  have  been  ascertained  subsequently  to  the  reading 
of  the  paper. 
