﻿312 
  Mr. 
  L. 
  T. 
  Hogben. 
  

  

  phase 
  in 
  animals 
  has 
  been 
  supplied 
  by 
  authors 
  who 
  have 
  been 
  more 
  con- 
  

   cerned 
  with 
  the 
  chromosome 
  complex 
  in 
  the 
  germ 
  cycle, 
  it 
  is 
  perhaps 
  

   pertinent 
  to 
  ask 
  whether 
  the 
  " 
  synizesis 
  " 
  figures 
  described 
  again 
  and 
  again 
  

   (as 
  in 
  a 
  previous 
  study 
  on 
  Hymenoptera) 
  would 
  not 
  present 
  a 
  less 
  homo- 
  

   geneous 
  appearance 
  with 
  more 
  specialised 
  technique. 
  It 
  is 
  certain 
  at 
  least 
  

   that 
  in 
  the 
  case 
  under 
  consideration, 
  it 
  is 
  easy 
  enough 
  at 
  the 
  point 
  of 
  

   maximal 
  contraction 
  to 
  obtain 
  typical 
  synaptic 
  knots 
  with 
  over 
  fixation 
  and 
  

   logwood 
  staining. 
  Maximal 
  contraction 
  does 
  not 
  seem 
  to 
  extend 
  over 
  a 
  

   long 
  period 
  in 
  P. 
  americana. 
  As 
  the 
  loops 
  widen 
  out, 
  the 
  interspace 
  between 
  

   the 
  constituent 
  halves 
  of 
  the 
  thickened 
  more 
  chromatic 
  loops 
  of 
  the 
  pachytene 
  

   stage 
  again 
  becomes 
  easily 
  visible 
  (fig. 
  13). 
  It 
  is 
  easy 
  to 
  see 
  both 
  in 
  this 
  

   and 
  the 
  preceding 
  stage 
  from 
  sections 
  at 
  right 
  angles 
  to 
  the 
  orientation 
  of 
  

   the 
  loops 
  (fig. 
  15) 
  that 
  the 
  reduced 
  number 
  of 
  loops 
  is 
  now 
  present. 
  The 
  

   longitudinally 
  divided 
  diplotene 
  loops 
  become 
  considerably 
  elongated, 
  and 
  

   enter 
  into 
  contact 
  with 
  the 
  nuclear 
  membrane 
  ; 
  but 
  again 
  become 
  shortened 
  

   somewhat 
  before 
  the 
  complete 
  dissolution 
  of 
  the 
  bouquet 
  figure 
  (figs. 
  14 
  

   and 
  16). 
  

  

  Up 
  to 
  this 
  point 
  the 
  facts 
  briefly 
  stated 
  are 
  as 
  follow 
  : 
  there 
  is 
  no 
  cleavage 
  

   of 
  chromosomes 
  in 
  the 
  telophase 
  of 
  the 
  oogonial 
  mitosis 
  ; 
  the 
  reduced 
  number 
  

   of 
  thickened 
  loops 
  in 
  the 
  stage 
  of 
  maximal 
  contraction 
  is 
  brought 
  about 
  by 
  the 
  

   parallel 
  conjugation 
  of 
  leptotene 
  threads 
  equivalent 
  in 
  number 
  to 
  the 
  telophasic 
  

   chromosomes 
  of 
  the 
  preceding 
  mitoses. 
  Provided 
  that 
  such 
  an 
  interpreta- 
  

   tion 
  is 
  not 
  incompatible 
  with 
  the 
  observed 
  transformation 
  of 
  diplotene 
  

   loops 
  into 
  heterotype 
  chromosomes, 
  it 
  is 
  difficult 
  to 
  escape 
  the 
  conclusion 
  that 
  

   the 
  process 
  described 
  is 
  not 
  analogous 
  in 
  any 
  way 
  to 
  a 
  premeiotic 
  prophase, 
  but 
  

   an 
  event 
  sui 
  generis 
  in 
  which 
  the 
  conjugation 
  of 
  homologous 
  chromosomes 
  is 
  

   effected. 
  It 
  is 
  therefore 
  necessary 
  now 
  to 
  enquire 
  whether 
  such 
  a 
  conclusion 
  

   is 
  consonant 
  with 
  the 
  subsequent 
  events. 
  But 
  before 
  doing 
  so, 
  it 
  may 
  be 
  

   stated 
  that 
  there 
  was 
  no 
  evidence 
  of 
  the 
  twisting 
  of 
  the 
  filaments 
  in 
  the 
  

   zygotene 
  stage 
  ; 
  indeed, 
  while 
  such 
  a 
  procedure 
  does 
  not 
  seem 
  improbable 
  in 
  

   the 
  case 
  of 
  leptotene 
  filaments 
  that 
  are 
  polarised 
  at 
  one 
  extremity 
  only, 
  it 
  is 
  

   difficult 
  to 
  imagine 
  in 
  the 
  case 
  of 
  paired 
  loops, 
  apparently 
  attached 
  at 
  both 
  

   ends, 
  any 
  mechanism 
  by 
  which 
  such 
  twisting 
  could 
  be 
  achieved. 
  The 
  union 
  

   of 
  the 
  constituent 
  halves 
  of 
  the 
  pachytene 
  loops 
  is 
  sufficiently 
  intimate 
  to 
  

   allow 
  the 
  possibility 
  of 
  " 
  crossing 
  over 
  " 
  ; 
  but 
  the 
  facts 
  do 
  not 
  seem 
  to 
  justify 
  

   the 
  belief 
  that 
  this 
  is 
  brought 
  about 
  by 
  such 
  means 
  as 
  are 
  postulated 
  by 
  the 
  

   chiasmatype 
  theory. 
  

  

  The 
  free 
  diplotene 
  loops 
  represented 
  in 
  fig. 
  16 
  rapidly 
  become 
  abbreviated 
  

   and 
  straightened, 
  a 
  process 
  in 
  which 
  two, 
  however, 
  lag 
  behind 
  their 
  fellows 
  

   precisely 
  as 
  do 
  two 
  of 
  the 
  loops 
  in 
  the 
  heterotype 
  prophase 
  of 
  the 
  male 
  sex 
  

  

  