﻿236 
  O'Neal: 
  Microsporogenesis 
  in 
  Datura 
  Stra:!^ioxium 
  

  

  1 
  

  

  strand 
  has 
  a 
  tendency 
  to 
  become 
  arranged 
  in 
  loose 
  loops, 
  Avhich 
  

   are 
  cut 
  off 
  to 
  form 
  bivalents. 
  In 
  this 
  behavior 
  Datura 
  is 
  strikingly 
  

   similar 
  to 
  Liliitm 
  as 
  described 
  by 
  Mottier 
  (9). 
  Fig. 
  7 
  represents 
  

   an 
  early 
  segmentation 
  stage. 
  It 
  may 
  be 
  observed 
  from 
  this 
  

   figure 
  that 
  all 
  of 
  the 
  bivalents 
  are 
  not 
  formed 
  by 
  the 
  cutting 
  off 
  of 
  

   loops 
  from 
  the 
  spirem 
  thread 
  but 
  that 
  some 
  of 
  them 
  must 
  arise 
  

   from 
  the 
  straight 
  loopless 
  part 
  of 
  it. 
  Shortly 
  before 
  the 
  seg- 
  

   mentation 
  of 
  the 
  chromosomes 
  is 
  brought 
  about, 
  the 
  double 
  

   nature 
  of 
  the 
  thread 
  becomes 
  evident 
  for 
  the 
  first 
  time 
  (Fig. 
  ii). 
  

   When 
  segmentation 
  is 
  complete 
  the 
  twelve 
  bivalent 
  chromosomes 
  

   may 
  be 
  made 
  out. 
  These 
  bivalents 
  may 
  assume 
  a 
  number 
  of 
  

   different 
  shapes 
  as 
  shown 
  in 
  Figs. 
  9 
  and 
  10. 
  Some 
  of 
  them 
  appear 
  

   as 
  closed 
  rings, 
  others 
  as 
  U's, 
  and 
  still 
  others 
  have 
  the 
  chromosomes 
  

  

  r 
  

  

  ft 
  

  

  twisted 
  about 
  each 
  other. 
  The 
  method 
  of 
  forming 
  these 
  various 
  

  

  V 
  

  

  types 
  is 
  easily 
  seen. 
  The 
  loops 
  Cut 
  off 
  from 
  the 
  spirem 
  (Fig. 
  7) 
  

   form 
  the 
  rings 
  by 
  the 
  union 
  or 
  overfapping 
  of 
  their 
  ends. 
  If 
  the 
  

   ends 
  are 
  hot 
  united 
  or 
  overlapped 
  the 
  U-shaped 
  form 
  results, 
  

   while 
  the 
  twisted 
  form 
  of 
  bivalent 
  results 
  from 
  the 
  contraction 
  of 
  a 
  

   loop 
  with 
  a 
  twist 
  in 
  it. 
  Payne 
  (12) 
  has 
  shown 
  that 
  in 
  the 
  European 
  

   earwig 
  ring-shaped 
  chromosomes 
  may 
  arise 
  in 
  three 
  different 
  

   ways 
  in 
  the 
  same 
  individual 
  or 
  even 
  in 
  the 
  sam'e 
  cell. 
  So 
  far 
  as 
  it 
  

   has 
  been 
  possible 
  to 
  determine, 
  the 
  method 
  given 
  above 
  is 
  the 
  

   only 
  one 
  followed 
  in 
  Datura, 
  In 
  Agave 
  (14) 
  Schaffner 
  was 
  able 
  

   to 
  find 
  a 
  constant 
  number 
  of 
  the 
  various 
  types 
  of 
  chromosomes 
  

   present. 
  A 
  careful 
  consideration 
  of 
  the 
  cells 
  of 
  Datura 
  fails 
  to 
  

   show 
  any 
  regularity 
  in 
  this 
  respect. 
  

  

  Preceding 
  the 
  metaphase 
  the 
  bivalents 
  appear 
  in 
  the 
  multipolar 
  

   arrangement 
  so 
  often 
  described. 
  This 
  stage 
  is 
  of 
  very 
  short 
  

   duration 
  and 
  difficulty 
  is 
  experienced 
  in 
  securing 
  it. 
  In 
  the 
  second 
  

   division 
  it 
  is 
  much 
  more 
  evident 
  (Figs. 
  16, 
  17). 
  In 
  the 
  meta- 
  

   phase 
  the 
  bivalents 
  become 
  arranged 
  in 
  the 
  usual 
  fashion 
  in 
  the 
  

   middle 
  of 
  the 
  cell. 
  In 
  a 
  polar 
  view 
  they 
  are 
  seen 
  to 
  have 
  a 
  regular 
  

   distribution 
  (FiG. 
  13). 
  In 
  this 
  stage 
  the 
  chromosomes 
  appear 
  more 
  

   or 
  less 
  heart 
  shaped 
  when 
  viewed 
  from 
  the 
  side 
  (FiG. 
  1 
  2) 
  and 
  closely 
  

   resemble 
  Mottier's 
  figure 
  for 
  Helleborus 
  (10). 
  There 
  is 
  no 
  separ- 
  

   ation 
  of 
  the 
  two 
  halves 
  of 
  the 
  chromosomes 
  on 
  the 
  way 
  to 
  the 
  

   poles, 
  although 
  the 
  longitudinal 
  division 
  has 
  doubtless 
  taken 
  place 
  

   (Fig. 
  ii). 
  Fig. 
  14 
  shows 
  a 
  telophase 
  stage 
  with 
  the 
  chromosomes 
  

  

  