﻿442 
  J. 
  W. 
  JEMKINSON. 
  

  

  can 
  liardly 
  be 
  doubted 
  from 
  the 
  work 
  of 
  van 
  der 
  Striclit 
  (1892) 
  

   and 
  Brans 
  that 
  such 
  exists 
  in 
  segmentation 
  stages. 
  

  

  On 
  another 
  small 
  point 
  I 
  must 
  disagree 
  with 
  Michaelis. 
  He 
  

   says 
  there 
  is 
  a 
  segmentation 
  nucleus. 
  I 
  find, 
  on 
  the 
  contrary, 
  

   in 
  some 
  preparations 
  of 
  fertiHsation 
  spindles 
  of 
  Triton 
  which 
  I 
  

   have, 
  that 
  there 
  are 
  two 
  distinct 
  sets 
  of 
  chromosomes. 
  At 
  the 
  

   same 
  time 
  we 
  ought 
  to 
  bear 
  in 
  mind 
  Boveri^s 
  (1890) 
  assertion 
  

   that 
  in 
  one 
  and 
  the 
  same 
  species 
  of 
  Echinus 
  there 
  is 
  a 
  

   variation 
  in 
  this 
  respect. 
  

  

  III. 
  Historical 
  and 
  Critical. 
  

  

  A, 
  Maturation. 
  

  

  1. 
  Structure 
  of 
  the 
  polar 
  spindles. 
  

  

  In 
  a 
  series 
  of 
  elaborate 
  and 
  valuable 
  memoirs 
  Carnoy 
  and 
  

   Le 
  Brun 
  have 
  described 
  the 
  formation 
  of 
  the 
  polar 
  spindles 
  

   and 
  bodies 
  in 
  both 
  Anurous 
  and 
  Urodelous 
  Amphibia. 
  Their 
  

   observations 
  arc 
  very 
  complete 
  and 
  detailed, 
  but 
  do 
  not 
  differ 
  

   in 
  any 
  other 
  important 
  respect 
  from 
  my 
  own. 
  

  

  The 
  first 
  polar 
  spindle 
  is 
  of 
  intra-nuclear 
  origin, 
  arising 
  

   from 
  a 
  special 
  portion 
  of 
  the 
  germinal 
  vesicle 
  — 
  the 
  " 
  plage 
  

   f 
  usoriale." 
  Both 
  first 
  and 
  second 
  polar 
  spindles 
  are 
  described 
  

   and 
  figured 
  with 
  inner 
  or 
  bi-polar 
  and 
  outer 
  or 
  mantle 
  fibres. 
  

   In 
  many 
  cases, 
  especially 
  in 
  the 
  early 
  stages 
  of 
  their 
  for- 
  

   mation, 
  the 
  poles 
  are 
  surrounded 
  by 
  astral 
  radiations. 
  The 
  

   authors 
  fail 
  to 
  find 
  any 
  centrosome 
  beyond 
  the 
  somewhat 
  

   indefinite 
  body 
  into 
  which 
  the 
  spindle 
  fibres 
  converge. 
  But 
  

   that 
  Carnoy 
  regards 
  this 
  body, 
  as 
  I 
  do 
  also, 
  as 
  a 
  physiological 
  

   centre, 
  seems 
  to 
  follow 
  from 
  his 
  remark 
  that 
  some 
  substance 
  

   comes 
  from 
  the 
  nucleus 
  — 
  " 
  cpii 
  agit 
  sur 
  le 
  reseau 
  et 
  y 
  produit 
  

   les 
  memes 
  irradiations 
  que 
  si 
  ces 
  substances 
  provenaient 
  d'un 
  

   centrosome 
  veritable." 
  

  

  In 
  the 
  Trout, 
  according 
  to 
  Behrens, 
  the 
  maturation 
  spindles 
  

   have 
  this 
  same 
  structure. 
  In 
  Amphioxus 
  (Sobotta, 
  1897) 
  

   only 
  the 
  second 
  polar 
  spindle 
  is 
  provided 
  with 
  mantle 
  fibres, 
  

   while 
  in 
  the 
  Mouse 
  (Sobotta, 
  1895) 
  these 
  fibres 
  are 
  absent 
  in 
  

   both 
  the 
  first 
  and 
  second. 
  

  

  