﻿[3] 
  DEVELOPMENT 
  OF 
  OSSEOUS 
  FISHES. 
  491 
  

  

  " 
  Subgerminale 
  Platte'''' 
  of 
  Kupffer; 
  Dotterhaut 
  and 
  membrana 
  vitellina 
  of 
  

   (Ellacher; 
  couche 
  intermediaire 
  of 
  V&u 
  Bambeke; 
  intermediary 
  layer 
  of 
  

   authors; 
  parablast 
  of 
  His, 
  Waldeyer, 
  Hoffmann, 
  Gasser, 
  Kupffer: 
  yelk- 
  

   hypoblast 
  of 
  the 
  writer; 
  couche 
  hcematogene 
  of 
  Vogt; 
  and 
  the 
  plasmedium 
  

  

  of 
  Eauber. 
  

  

  The 
  annular 
  thickening 
  of 
  the 
  periblast, 
  just 
  under 
  the 
  edge 
  of 
  tbe 
  

   blastodisk, 
  is 
  clearly 
  homologous 
  with 
  the 
  Keimwall 
  of 
  His 
  or 
  the 
  Keim- 
  

   wulst 
  of 
  Kolliker, 
  as 
  seen 
  in 
  the 
  ova 
  of 
  Sauropsida. 
  The 
  function 
  of 
  

   this 
  periblast 
  is 
  also 
  clearly 
  established 
  throughout 
  the 
  various 
  series 
  

   of 
  Vertebrates 
  which 
  develop 
  meroblastic 
  ova; 
  its 
  cells, 
  in 
  fact, 
  in- 
  

   corporate 
  yelk 
  particles, 
  by 
  a 
  process 
  which 
  is 
  essentially 
  one 
  approx- 
  

   imating 
  that 
  of 
  intracellular 
  digestion. 
  Finally, 
  the 
  periblast 
  may 
  give- 
  

   rise 
  directly 
  to 
  free 
  cells, 
  which 
  pass 
  into 
  the 
  vascular 
  channels 
  of 
  the 
  

   embryo 
  as 
  blood-corpuscles. 
  

  

  This 
  disposition 
  to 
  absorb 
  the 
  underlying 
  quiescent 
  plasma 
  is 
  also 
  

   shown 
  by 
  the 
  lower 
  cells 
  of 
  the 
  true 
  hypoblast 
  which 
  immediately 
  over- 
  

   lies 
  the 
  periblast. 
  Such 
  hypoblastic 
  cells 
  which 
  are 
  larger 
  than 
  their 
  

   neighbors 
  have 
  been 
  called 
  megasphsera 
  by 
  His, 
  aud 
  have 
  also 
  been 
  

   figured 
  by 
  Kolliker 
  in 
  the 
  Avian 
  blastodisk, 
  while 
  they 
  have 
  been 
  en- 
  

   countered 
  by 
  the 
  writer 
  in 
  the 
  blastodisks 
  of 
  Teleosts. 
  

  

  In 
  the 
  eggs 
  of 
  the 
  cod, 
  as 
  in 
  most 
  pelagic 
  fish 
  ova, 
  the 
  periblast 
  is 
  

   quite 
  thin 
  after 
  the 
  closure 
  of 
  the 
  blastopore, 
  but 
  as 
  the 
  yelk 
  diminishes 
  

   in 
  quantity 
  with 
  the 
  progress 
  of 
  development 
  this 
  layer 
  becomes 
  de- 
  

   cidedly 
  thicker. 
  The 
  entire 
  yelk 
  is, 
  in 
  fact, 
  first 
  converted 
  into 
  the 
  

   Plasmodia! 
  substance 
  of 
  the 
  periblast 
  before 
  it 
  is 
  absorbed 
  by 
  other 
  

   parts 
  of 
  the 
  embryo. 
  It 
  therefore 
  results 
  that 
  the 
  last 
  portion 
  of 
  the 
  

   yelk 
  to 
  disappear 
  is 
  the 
  periblast. 
  

  

  The 
  periblast 
  is 
  undoubtedly 
  hypoblastic 
  in 
  position, 
  and 
  in 
  many 
  

   large-yelked 
  forms 
  is 
  homologous 
  with 
  the 
  splanchnopleure, 
  as 
  in 
  the 
  

   case 
  of 
  the 
  embryos 
  of 
  Salmonoids, 
  in 
  which, 
  together 
  with 
  the 
  vascular 
  

   network 
  traversing 
  it 
  superficially, 
  it 
  eventually 
  occupies 
  the 
  position 
  

   of 
  the 
  splanchnopleural 
  mesoblast 
  in 
  relation 
  to 
  the 
  other 
  layers. 
  

  

  (I) 
  Development 
  of 
  the 
  hypoblast 
  or 
  the 
  gastrulation 
  of 
  the 
  egg. 
  — 
  This 
  

   almost 
  threadbare 
  subject 
  I 
  return 
  to 
  reluctautly, 
  because 
  so 
  much 
  

   which 
  is 
  erroneous 
  has 
  been 
  written 
  about 
  it. 
  Balfour's 
  account 
  in 
  his 
  

   Comparative 
  Embryology, 
  ii, 
  57, 
  is 
  far 
  from 
  clear, 
  and 
  conveys 
  but 
  

   little 
  definite 
  information 
  as 
  to 
  the 
  origin 
  of 
  the 
  hypoblast. 
  That 
  the 
  

   latter 
  is 
  developed 
  as 
  a 
  "centripetal 
  ingrowth 
  of 
  cells 
  from 
  the 
  margin 
  

   of 
  the 
  blastodisk" 
  (Agassiz 
  and 
  Whitman) 
  there 
  can 
  no 
  longer 
  be 
  any 
  

   doubt, 
  though 
  in 
  my 
  earlier 
  studies 
  on 
  the 
  development 
  of 
  the 
  cod 
  I 
  

   opposed 
  that 
  view, 
  because 
  I 
  had 
  not 
  succeeded 
  in 
  witnessing 
  the 
  pro- 
  

   cess, 
  which 
  I 
  have 
  observed 
  since 
  in 
  detail, 
  as 
  has 
  also 
  beeu 
  done 
  by 
  

   Cunningham. 
  

  

  I 
  find 
  that 
  the 
  cells 
  at 
  the 
  edge 
  of 
  the 
  blastodisk 
  are 
  inflected 
  around 
  

   its 
  entire 
  margin, 
  but 
  that 
  at 
  the 
  point 
  where 
  the 
  future 
  embryo 
  is 
  

   formed 
  the 
  ingrowth 
  is 
  most 
  rapid, 
  and 
  soon 
  becomes 
  somewhat 
  wider. 
  

  

  