628 
ME.  J.  LISTEE  oy  THE  C^TA^^:O^S 
tents  of  the  cells  are  described  as  dark  granules  suspended  in  a fluid ; and  both  vox 
WiTTiCH  and  Haeless  have  distinctly  seen  the  granules  roUing  along  in  the  offsets 
during  the  process  of  concentration.  All  the  authorities  agree  in  the  opinion  that  the 
fluid  and  granules  move  together  from  one  part  of  the  cell  to  another,  the  oflfeets  being 
supposed  empty  of  both  when  the  pigment  is  accumulated  in  the  body  of  the  cell*. 
In  some  respects  the  above  description  agrees  with  my  own  experience  of  the  common 
frog  of  this  country  [Bana  temporaria).  I find  that  this  well-known  animal  exhibits 
changes  of  hue  almost  as  great  as  those  of  the  chameleon,  evei’y  specimen  being  capable 
of  varying  from  a very  pale  to  a very  dark  colom’,  the  former  being  generally  greenish 
yellow,  but  in  some  varieties  reddish ; and  the  latter  brownish  black,  or  sometimes 
coal  black;  while  between  these  extremes  any  intermediate  shade  may  be  assumed. 
The  depth  of  tint  is  generally  proportioned  to  that  of  suiTounding  objects:  thus  a frog 
caught  in  a recess  in  a black  rock  was  itself  almost  black ; but  after  it  had  been  kept  for 
about  an  hour  on  white  flagstones  in  the  sun,  was  found  to  be  dusky  yellow,  vrith  dark 
spots  here  and  there.  It  was  then  placed  again  in  the  hollow  of  the  rock,  and  in  a 
quarter  of  an  hour  had  resumed  its  former  darkness.  These  effects  are  independent  of 
changes  of  temperature ; for  similar  results  may  be  obtained  by  placing  a fi’og  alternately 
in  a vessel  from  which  luminous  rays  are  excluded,  and  in  a white  earthen  jar  covered 
with  glass,  in  the  same  situation.  Different  examples,  however,  differ  much  in  then- 
sensitiveness  to  light.  A violent  struggle  on  the  part  of  the  animal  is  often  followed  by 
a speedy  alteration  from  a dark  to  a pale  state  of  the  skin.  It  seems  ver}*  doubtful 
whether  psychical  excitement  has  anything  to  do  with  this  occurrence,  any  more  than 
with  the  arterial  contraction  which  invariably  takes  place  under  such  cu-cumstances. 
Neither  oil  of  turpentine  nor  galvanism,  when  applied  to  the  integument,  produces,  so 
far  as  I have  seen,  any  effect  upon  its  colom- ; our  species  being  httle  influenced  in  this 
respect  by  direct  irritation.  I have  however  frequently  observed,  after  forcibly  pinching 
a dark  web,  that  a pale  ring,  about  i^th  of  an  inch  in  breadth,  has  formed  around  the 
area  so  treated ; but  this  was  very  slow  in  appearing,  being  first  noticed  fi’om  half  an 
hour  to  an  hour  after  the  pinch  was  given. 
The  webs  of  the  hind  feet,  examined  under  a low  power  of  the  microscope,  exhibit 
differences  in  the  distribution  of  the  dark  pigment f according  to  the  tint  of  the  skin. 
* From  the  way  in  which  vox  Wittich  alludes  to  Beucke’s  description,  it  is  clear  that  the  latter  supposed 
the  cells  to  he  contractile.  Vox  Wittich  himself  in  his  first  paper  speaks  of  the  movement  of  the  pigment 
induced  by  galvanism  as  “satisfactorily”  showing  “that  the  stellate  pigment-cells  ai-e  contractile.”  In  his 
second  paper  (vide  Mullee’s  Archiv,  1854,  p.  263),  he  expresses  some  doubt  regarding  the  contractilitT  of 
the  cell-wall,  but  clearly  speaks  of  the  contents  (fiuid  and  granules)  as  moving  together.  Haeless,  after 
describing  “ the  roUmg  of  the  pigment-molecules  towards  the  centre  of  the  cell,”  goes  on  to  say,  “ that  this 
rolling  may  be  possible,  there  must  be  a fiuid  in  the  cells  and  ofisets,  to  xoliicli  the  molecules  owe  their  move- 
ment.” He  takes  it  for  granted  that  the  movement  of  the  fiuid  must  be  due  to  some  contractile  agency,  and 
as  he  finds  no  apparatus  of  this  natme  around  the  cells,  and  as  the  unstriped  muscular  fibres  of  the  skin 
have  no  special  relation  to  them,  he  infers  that  the  cell-wall  is  itself  contractile. 
t Other  kinds  of  pigment  are  also  present  in  the  skin  of  the  common  frog,  generally  of  3'ellow  colour, 
