﻿528 
  REPORT 
  OF 
  COMMISSIONER 
  OF 
  FISH 
  AND 
  FISHERIES. 
  [40] 
  

  

  The 
  myotomes 
  gradually 
  increase 
  in 
  number, 
  and 
  at 
  the 
  time 
  the 
  blas- 
  

   topore 
  closes 
  but 
  three 
  or 
  four 
  are 
  visible, 
  as 
  shown 
  in 
  fig. 
  120. 
  Some- 
  

   what 
  later 
  more 
  are 
  added 
  behind 
  those 
  already 
  formed, 
  as 
  shown 
  in 
  

   figs. 
  98, 
  101, 
  102, 
  103, 
  123, 
  126, 
  127, 
  until 
  the 
  full 
  complement 
  is 
  devel- 
  

   oped, 
  as 
  shown 
  in 
  fig. 
  151. 
  • 
  

  

  Of 
  the 
  sense 
  organs, 
  the 
  optic 
  lobes, 
  from 
  which 
  the 
  eyes 
  are 
  formed, 
  

   are 
  the 
  first 
  to 
  become 
  developed. 
  These 
  appear 
  as 
  a 
  pair 
  of 
  lateral 
  

   elongated 
  thickenings 
  of 
  the 
  epiblast 
  at 
  the 
  front 
  end 
  of 
  the 
  embryonic 
  

   axis. 
  Four 
  stages 
  of 
  the 
  differentiation 
  of 
  the 
  optic 
  lobes 
  are 
  shown, 
  

   as 
  viewed 
  from 
  the 
  above, 
  in 
  figs. 
  114 
  to 
  117. 
  These 
  phases 
  of 
  the 
  de- 
  

   velopment 
  Of 
  the 
  eyes 
  and 
  head 
  may 
  be 
  readily 
  connected 
  with 
  the 
  more 
  

   advanced 
  ones 
  represented 
  in 
  figs. 
  113 
  and 
  128. 
  

  

  The 
  gradual 
  extension 
  of 
  the 
  cleavage 
  cavity 
  sc 
  under 
  the 
  growing 
  

   blastodisk 
  may 
  be 
  traced 
  by 
  reference 
  to 
  figs. 
  137, 
  138, 
  139, 
  140, 
  141, 
  118, 
  

   119, 
  120, 
  121, 
  98, 
  102, 
  103, 
  122, 
  126, 
  and 
  127. 
  After 
  the 
  entire 
  yelk 
  is 
  cov- 
  

   ered 
  by 
  the 
  blastoderm 
  the 
  only 
  cellular 
  membranous 
  covering 
  invest- 
  

   ing 
  it 
  externally 
  is 
  the 
  extremely 
  thin 
  epiblastic 
  membrane 
  e, 
  shown 
  

   in 
  section, 
  fig. 
  121, 
  and 
  between 
  this 
  and 
  the 
  periblast 
  p 
  of 
  the 
  same 
  

   figure 
  the 
  cleavage 
  cavity 
  sc 
  is 
  included 
  ; 
  this 
  again 
  being 
  directly 
  con- 
  

   tinuous 
  with 
  the 
  body 
  cavity 
  be 
  on 
  either 
  side 
  of 
  the 
  intestine 
  i. 
  This 
  

   relation 
  of 
  the 
  surrounding 
  parts 
  to 
  the 
  cleavage 
  cavity 
  demonstrates 
  

   very 
  conclusively 
  that 
  the 
  membrane 
  e, 
  in 
  fig. 
  121, 
  must 
  be 
  the 
  sorna- 
  

   topleure, 
  while 
  the 
  periblast 
  p 
  must 
  undoubtedly 
  be 
  homologized 
  with 
  

   the 
  splanchnopleure. 
  The 
  vascular 
  network 
  developed 
  over 
  the 
  peri- 
  

   blast 
  and 
  in 
  intimate 
  connection 
  with 
  it, 
  in 
  the 
  ova 
  of 
  many 
  species 
  of 
  

   fishes, 
  is 
  also 
  splanchnopleural 
  and 
  homologous 
  with 
  the 
  area 
  vasculosa 
  

   or 
  omphalomeseraic 
  mesh 
  work 
  developed 
  over 
  the 
  yelk 
  of 
  higher 
  forms. 
  

   Of 
  the 
  correctness 
  of 
  this 
  homology 
  I 
  think 
  there 
  can 
  scarcely 
  be 
  any 
  

   doubt 
  whatever. 
  

  

  The 
  yelk 
  of 
  Clupea, 
  and 
  of 
  Teleost 
  embryos 
  generally, 
  it 
  may 
  therefore 
  

   be 
  said, 
  is 
  intra-abdominal 
  ; 
  it 
  is 
  excluded 
  from 
  direct 
  connection 
  with 
  

   the 
  intestine, 
  but 
  remains 
  adherent 
  to 
  it 
  for 
  a 
  considerable 
  time 
  by 
  its 
  

   inferior 
  face, 
  through 
  the 
  intermediation 
  of 
  the 
  periblast 
  immediately 
  

   underlying 
  the 
  intestine. 
  The 
  yelk 
  is 
  almost 
  naked 
  were 
  it 
  not 
  for 
  the 
  

   thin 
  syncytium, 
  known 
  as 
  the 
  periblast, 
  and 
  which, 
  on 
  the 
  lower 
  and 
  

   lateral 
  portions 
  of 
  the 
  yelk, 
  can 
  hardly 
  be 
  considered 
  to 
  rank 
  as 
  a 
  true 
  

   membrane 
  on 
  account 
  of 
  the 
  widely- 
  scattered 
  or 
  diffused 
  nuclei 
  it 
  con- 
  

   tains. 
  The 
  splanchnopleure 
  (= 
  periblast) 
  of 
  osseous 
  fishes 
  is 
  therefore 
  

   rudimentary 
  or 
  very 
  feebly 
  developed 
  over 
  the 
  vitellus, 
  so 
  that 
  the 
  lat- 
  

   ter 
  may 
  be 
  considered 
  to 
  be 
  intra-abdominal, 
  not 
  only 
  because 
  there 
  

   is 
  no 
  umbilical 
  stalk 
  developed, 
  but 
  also 
  because 
  it 
  is 
  very 
  imperfectly 
  

   inclosed 
  by 
  the 
  splanchnopleure. 
  

  

  The 
  chorda 
  is 
  unusually 
  well 
  developed 
  in 
  Glupea 
  and 
  forms 
  a 
  mas- 
  

   sive 
  axial 
  rod 
  at 
  the 
  time 
  of 
  hatching, 
  as 
  shown 
  in 
  cross-section 
  at 
  

   ch, 
  fig. 
  121, 
  and 
  for 
  its 
  whole 
  length 
  in 
  fig. 
  148. 
  Derived 
  as 
  already 
  

   stated 
  from 
  the 
  median 
  dorsal 
  part 
  of 
  the 
  hypoblast, 
  it 
  retains 
  its 
  con- 
  

  

  