PiaMENTAEY  SYSTEM  OF  THE  FEOO. 
629 
such  as  will  be  understood  by  referring  to  Plate  XLVII.,  where  fig.  1 is  from  a dark 
portion  of  web,  and  fig.  2 from  a pale  part  in  the  same  animal.  In  fig.  2 the  colouring 
matter  is  seen  to  be  collected  in  black  spots  of  irregular  angular  shape.  This,  however, 
is  not  the  state  which  exists  when  the  colour  is  palest,  for  then  the  masses  of  pigment 
are  in  the  form  of  round  dots,  as  in  the  part  to  the  right  in  fig.  1,  Plate  XL VIII. 
Neither  does  fig.  1 of  Plate  XLVII.  give  the  condition  met  with  when  the  skin  is  dark- 
est, in  which  case  all  that  meets  the  eye  on  superficial  observation  is  a reticular  appear- 
ance, such  as  is  represented  in  the  stripe  down  the  middle  of  fig.  I,  Plate  XLVIII., 
and  in  the  lower  part  of  fig.  2 in  the  same  Plate.  When  the  colour  of  the  integument 
is  about  medium,  the  pigment  is  disposed  in  a truly  stellate  manner,  as  on  the  left  side 
of  fig.  I,  Plate  XLVIII.  It  may  be  convenient  for  the  purposes  of  description,  to 
designate  these  various  states  as  respectively  the  dotted,  angular,  stellate  and  reticular 
conditions  of  the  pigment. 
WTien  a higher  magnifying  power  is  applied  in  an  extremely  dark  state  of  the  skin, 
the  chromatophorous  cells,  for  such  they  seem  to  be,  appear  as  depicted  in  Plate  XLVII. 
fig.  3,  where  two  of  them  are  given,  along  with  an  adjacent  capillary  distended  with 
blood-corpuscles.  Each  cell  consists  of  a somewhat  flattened  central  part  with  several 
irregular  offsets,  of  considerable  diameter  near  the  central  part,  but  speedily  breaking 
up  into  small  branches.  The  ultimate  ramifications,  some  of  which  are  of  extreme 
minuteness,  anastomose  freely  with  one  another  and  with  those  of  neighbouring  cells, 
constituting  a very  delicate  and  close-meshed  network,  which  pervades  the  whole  thick- 
ness of  the  true  skin,  and  especially  follows  the  course  of  the  blood-vessels,  entering  into 
the  composition  of  the  cellular  coat  of  the  arteries  and  veins,  and  twining  about  the 
capillaries  in  a very  remarkable  manner.  The  walls  of  these  cells  and  of  their  tubular 
offsets  appear  to  be  extremely  delicate,  and  some  attempts  which  I have  made  to  isolate 
them  from  surrounding  tissues  have  barely  served  to  demonstrate  their  existence.  The 
cells  vary  considerably  in  dimensions  according  to  the  size  of  the  animal ; thus,  those  in 
figs.  8,  9 and  10,  which  are  from  young  frogs,  though  magnified  500  diameters,  show  in 
the  drawing  even  smaller  than  those  in  fig.  3,  magnified  only  250  times,  the  latter  being 
from  a full-grown  specimen.  In  an  average  full-sized  cell  of  a large  frog,  the  middle 
portion  was  found  to  measure  3-^th  of  an  inch  in  length  by  e-fo^fr  of  iiich  in 
breadth,  and  j-^^th  of  an  inch  in  thickness.  The  last-named  dimension  was  obtained 
by  carrying  the  focus  of  an  object-glass  of  high  power,  from  the  most  superficial  to  the 
deepest  part  by  the  screw  for  giving  slow  motion,  and  reading  off  on  its  graduated  circle 
but  sometimes  red.  My  attention  has  not  been  much  directed  to  these,  but  I have  noticed  that  they  are 
contaiued  in  receptacles  of  the  same  general  form  and  structure  as  those  which  hold  dark  pigment ; and  on 
one  occasion,  since  the  reading  of  the  paper,  I observed  the  colouring  matter  disposed  in  a stellate  manner 
with  complex  ramifications  in  one  part  of  a web,  and  in  another  part  collected  into  round  spots ; implying 
that  these  cells  possess  the  functions  of  concentration  and  diffusion  of  the  pigment.  They  do  not,  however, 
always  act  in  harmony  with  the  dark  cells ; and  it  is  probably  through  their  agency  that  changes  in  tint, 
such  as  I have  seen  to  occur  in  one  and  the  same  frog,  independent  of  mere  lightness  and  darkness  of  shade, 
are  produced. 
