PI&MENTAEY  SYSTEM  OF  THE  FEOG. 
631 
not  flat,  but  subcylindrical.  But  the  difierences  in  tint  are  sufficiently  accounted  for  by 
the  circumstance  that  in  the  dark  branches  the  colouring  particles  are  closely  packed 
together,  whereas  in  the  bodies  of  the  cells  and  the  paler  parts  of  the  offsets,  the  indi- 
vidual granules  are  separated  from  one  another  by  considerable  colourless  intervals. 
Hence  it  is  clear  that  the  degree  of  darkness  of  any  part  of  a cell  does  not  depend  so 
much  on  the  bulk  of  its  contents  in  the  aggregate,  as  on  the  proportion  which  the  pig- 
ment molecules  in  it  bear  to  the  fluid  in  which  they  are  suspended. 
If  the  whole  contents  of  the  processes  were  forced  into  the  central  parts  during  con- 
centration of  the  pigment,  and  driven  back  again  during  diffusion,  the  bodies  of  the  cells 
would  be  subject  to  great  variations  in  capacity,  becoming  turgid  in  concentration  and 
collapsed  in  diffusion ; and  the  bulk  of  the  central  coloured  mass  would  be  great  in  the 
former  case,  but  small  in  the  latter.  The  very  reverse,  however,  really  takes  place. 
Fig.  6 represents  the  appearance  of  the  pigment  in  a concentrated  condition,  in  one  of 
the  same  cells  which  in  fig.  3 show  it  in  full  diffusion.  During  the  time  in  which  this 
change  took  place,  the  adjacent  capillary  had  shrunk  to  about  half  its  former  size,  but 
it  will  be  recognised  by  its  general  form,  and  will  indicate  which  of  the  two  cells  is  that 
under  consideration.  Both  the  figures  were  drawn  on  the  same  scale  with  the  camera 
lucida*,  so  that  accuracy  of  proportion  is  ensured.  The  circular  black  mass  into  which 
the  colouring  matter  is  now  all  collected,  measures  less  across  than  either  the  length  or 
breadth  of  the  body  of  the  cell  in  the  diffused  state  of  the  pigment.  Further,  the  mass 
is  not  spherical,  but  of  flattened  form,  and  its  thickness  is  only  about  that  of  the  central 
part  of  the  cell  in  diffusion.  This  we  know  from  the  appearances  presented  by  the 
spots  of  concentrated  pigment  in  other  cells  seen  edgewise,  as  is  the  case  with  some  in 
fig.  7,  which  represents  the  outline  of  the  wall  of  a large  blood-vessel,  and  the  pigment 
contained  in  its  external  coat  in  nearly  complete  concentration.  Hence  it  appears  that 
all  the  pigment-granules  contained  in  the  body  of  the  cell  and  the  minutely  ramifying 
processes  in  the  diffused  state,  have  been  brought  together  into  a space  considerably 
less  than  was  then  occupied  by  the  pale  contents  of  the  body  of  the  cell  alone.  The 
coloured  particles  have  been  concentrated  into  a dense  disciform  mass,  but  the  fluid  in 
which  they  were  suspended  has  been  left  behind. 
Fig.  4 shows  the  pigment  in  the  same  cells  as  fig.  3 in  an  intermediate  stage,  in  which 
the  process  of  concentration  is  about  half  accomplished ; the  upper  one  being  in  the  con- 
dition which  would  appear  stellate  under  a low  magnifying  power.  The  greater  part  of 
the  pigment  is  collected  in  the  bodies  of  the  cells,  especially  towards  their  central  parts : 
in  the  middle  of  each  dark  mass,  however,  is  a pale  spot,  doubtless  due  to  the  circum- 
stance of  the  granules  not  having  yet  insinuated  themselves  between  the  cell-wall  and  the 
nucleus,  which,  as  shown  above,  probably  lies  in  contact  with  it.  This  appearance  of 
pale  central  points  was  very  general  in  the  web  at  the  time  when  fig.  4 was  drawn,  but 
gradually  disappeared  as  the  aggregation  of  the  pigment-molecules  proceeded,  and  does 
* All  the  drawings  in  the  two  Plates  which  accompany  this  paper  were  made  with  the  assistance  of  this 
very  valuable  instrument. 
MDCCCLVIII.  4 0 
