﻿540 
  J, 
  BRETLA.ND 
  FARMKli 
  AND 
  J. 
  E. 
  S. 
  MOORE. 
  

  

  (heterotype) 
  division 
  in 
  these 
  fislies 
  are, 
  like 
  the 
  earlier 
  ones, 
  

   carried 
  out 
  in 
  the 
  same 
  manner 
  as 
  in 
  the 
  cockroach 
  itself. 
  

   There 
  seems 
  to 
  be 
  no 
  room 
  left 
  for 
  doubt 
  that 
  the 
  coarse 
  

   spirem 
  contracts 
  again 
  into 
  a 
  polarised 
  figure 
  and 
  that 
  the 
  

   loops 
  of 
  this 
  second 
  contraction 
  are 
  converted 
  directly 
  into 
  

   the 
  diaschistic 
  heterotype 
  chromosomes. 
  

  

  We 
  have 
  found 
  no 
  figures 
  which 
  in 
  any 
  way 
  militate 
  

   against 
  this 
  view 
  of 
  the 
  origin 
  of 
  the 
  heterotype 
  chromosomes 
  

   among 
  these 
  fishes; 
  and 
  the 
  apparent 
  reason 
  why 
  the 
  process 
  

   has 
  not 
  hitherto 
  been 
  apprehended 
  seems 
  to 
  be 
  that 
  among 
  

   elasmobranchs 
  the 
  second 
  contraction-figure, 
  or 
  synapsis, 
  

   is 
  much 
  more 
  rapid 
  than 
  in 
  the 
  cockroach. 
  Consequently 
  

   one 
  is 
  apt 
  to 
  pass 
  over 
  its 
  existence, 
  from 
  stages 
  corre- 
  

   sponding 
  to 
  that 
  represented 
  in 
  fig. 
  68 
  to 
  the 
  later 
  stage 
  

   given 
  in 
  fig. 
  73, 
  whereby 
  it 
  might 
  be 
  natural 
  to 
  conclude 
  

   that 
  the 
  heterotype 
  loop, 
  or 
  ring, 
  arose 
  from 
  the 
  opening 
  out 
  

   of 
  the 
  longitudinal 
  split 
  in 
  the 
  spirem 
  segments. 
  So 
  far, 
  

   then, 
  as 
  the 
  origin 
  of 
  the 
  reduced 
  number 
  of 
  heterotype 
  

   chromosomes 
  is 
  concerned, 
  we 
  reach, 
  after 
  a 
  renewed 
  study 
  of 
  

   the 
  process 
  in 
  elasmobranchs, 
  exactly 
  the 
  same 
  conclusion 
  

   as 
  we 
  did 
  in 
  the 
  case 
  of 
  the 
  cockroach 
  ; 
  that 
  is, 
  the 
  

   synaptic 
  and 
  pre-maiotic 
  prophases 
  in 
  the 
  origin 
  of 
  the 
  repro- 
  

   ductive 
  elements 
  in 
  these 
  widely 
  separated 
  animal 
  types 
  are 
  

   apparently 
  identical. 
  In 
  both, 
  the 
  reduction 
  of 
  the 
  number 
  of 
  

   chromosomes 
  is 
  brought 
  about 
  by 
  a 
  special 
  prophase, 
  wherein 
  

   pairs 
  of 
  longitudinally 
  split 
  somatic 
  chromosomes 
  become 
  

   temporarily 
  united 
  together, 
  and 
  afterwards 
  merely 
  separate 
  

   from 
  one 
  another 
  during 
  the 
  diaschistic 
  (heterotype) 
  division. 
  

  

  In 
  Elasmobranchs 
  the 
  later 
  phases 
  of 
  the 
  first 
  maiotic 
  

   mitosis 
  have 
  already 
  been 
  fully 
  described 
  by 
  one 
  of 
  us,^ 
  and 
  

   at 
  the 
  present 
  time 
  we 
  have 
  nothing 
  to 
  add 
  to 
  the 
  descrip- 
  

   tion 
  already 
  published. 
  With 
  respect 
  to 
  the 
  second 
  maiotic 
  

   division, 
  however, 
  it 
  is 
  now 
  necessary 
  to 
  append 
  some 
  

   correction 
  to 
  the 
  previous 
  description. 
  In 
  this 
  it 
  may 
  be 
  

   remembered 
  that 
  the 
  second 
  maiotic 
  or 
  homotype 
  division 
  

   was 
  described 
  as 
  having 
  the 
  same 
  characters 
  as 
  the 
  first 
  

   ' 
  J. 
  E. 
  S. 
  Moore, 
  loc. 
  cit,. 
  

  

  