PIGMENTAET  SYSTEM  OE  THE  EEOG. 
633 
it  unless  the  molecules  had  a special  tendency  towards  the  centre.  The  circular  form 
assumed  by  the  mass  of  pigment  when  concentration  is  complete,  is  strongly  suggestive 
of  a central  attractive  force  acting  on  the  granules.  The  occurrence  of  the  central  pale 
points,  which  are  represented  in  fig.  4,  showing  that  the  nucleus  was  there  in  the  middle 
of  the  concentrating  pigment,  led  me  at  first  to  suppose  that  this  body  was  the  attractive 
agent*.  I afterwards  took  pams  to  ascertain  whether  the  nucleus  always  has  this  rela- 
tion to  the  mass,  and  found  that  such  is  not  the  case.  On  the  22nd  of  October,  1857, 
I watched  three  adjacent  cells  during  the  process  of  ^ost  mortem  concentration ; in  two 
of  them  the  nucleus  ultimately  projected  by  about  a quarter  of  its  length  at  one  side 
from  the  black  spot,  while  in  the  other  cell  the  aggregated  molecules  covered  only  one- 
third  of  the  nucleus,  so  that  no  'part  of  that  body  lay  in  the  middle  of  the  mass.  The 
pomt  to  which  the  granules  appear  to  have  a special  tendency  is  the  middle  of  the  body 
of  the  cell,  which  seems  always  to  correspond  with  the  centre  of  the  disc  of  molecules, 
whereas  the  nucleus  is  often  excentrically  placed  in  the  cell. 
The  difiusion  of  the  molecules  is  not  merely  a passive  result  of  the  cessation  of  con- 
centration, as  has  been  hitherto  supposed.  In  watching  closely  the  occurrence  of  the 
phenomenon,  I have  seenf  the  granules  start  ofi*  suddenly  from  the  central  mass,  with  a 
velocity  which  implied  that  they  were  under  the  influence  of  forces  very  different  from 
those  which  cause  molecular  movements  in  them  when  shed  from  their  containing  cells. 
That  the  process  requires  the  vital  forces  of  the  cells  to  be  in  full  operation,  is  also 
proved  by  the  fact,  that  any  agency,  such  as  a galvanic  shock,  which  temporarily  para- 
lyses their  functions,  arrests  difiusion  as  well  as  concentration ; whereas,  if  the  former 
were  merely  passive,  it  would  take  place  as  soon  as  the  concentrating  power  was  set  at 
rest:{;. 
I have  already  pointed  out  the  sparsely  scattered  state  of  the  granules  in  the  central 
receptacles,  compared  with  their  accumulation  in  the  branches  of  the  offsets,  in  the  fully 
difiused  state  shown  in  fig.  3.  This  contrast  is  sometimes  much  more  striking,  so  that 
the  bodies  of  the  cells  are  almost  coloui’less,  and  require  some  experience  with  the  tissue 
in  order  to  detect  them.  This  indicates  a special  tendency  on  the  part  of  the  granules 
to  leave  the  middle  of  the  cell.  Yet  to  however  great  a degree  diffusion  be  carried, 
there  always  remain  some  molecules  in  the  body  of  the  cell  uniformly  distributed 
throughout  its  thickness  and  not  attached  to  the  parietes,  as  they  would  have  been  had 
their  dispersion  been  caused  by  attraction  on  the  part  of  the  cell-wall.  This  disposition 
of  the  granules,  which  obtains  even  in  the  immediate  vicinity  of  the  nucleus,  appears 
also  distinct  e\idence  against  the  operation  of  a central  repulsive  force ; for  this  would 
render  the  body  and  the  adjoining  parts  of  the  processes  as  clear  of  pigment  as  the 
remote  branches  are  made  in  concentration.  The  hypothesis  which  would  seem  most 
♦ Tliis  was  the  view  expressed  in  tbe  paper  as  it  was  read. 
■f  This  observation  was  made  after  the  reading  of  the  paper. 
f Eor  further  information  regarding  the  effects  of  various  agents  on  the  functions  of  the  pigment-cells, 
see  the  next  paper  in  this  volume. 
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