﻿492 
  REPORT 
  OF 
  COMMISSIONER 
  OF 
  FISH 
  AND 
  FISHERIES. 
  [4] 
  

  

  Later, 
  the 
  hypoblastic 
  layer, 
  which 
  has 
  arisen 
  by 
  the 
  process 
  of 
  inflec- 
  

   tion 
  just 
  described, 
  forms 
  a 
  sort 
  of 
  rounded 
  promontory 
  or 
  tongue 
  of 
  

   cells, 
  several 
  deep, 
  which 
  is 
  prolonged 
  inwards 
  under 
  the 
  epiblast 
  with 
  

   the 
  progress 
  of 
  the 
  development 
  and 
  the 
  growth 
  in 
  length 
  of 
  the 
  two- 
  

   layered 
  rudiment 
  of 
  the 
  embryo. 
  The 
  views 
  of 
  Gcette, 
  11 
  Haeckel, 
  12 
  Hen- 
  

   neguy, 
  13 
  Ziegler, 
  Kingsley 
  and 
  Oonn, 
  14 
  Agassiz 
  and 
  Whitman, 
  Brook, 
  

   and 
  of 
  Cunningham, 
  on 
  this 
  point, 
  agree 
  pretty 
  closely 
  as 
  to 
  the 
  main 
  

   fact 
  of 
  the 
  occurrence 
  of 
  a 
  marginal 
  inflection 
  of 
  the 
  blastodisk. 
  

  

  As 
  observed 
  by 
  a 
  number 
  of 
  investigators, 
  the 
  centripetal 
  inflection 
  

   of 
  the 
  margin 
  of 
  the 
  blastodisk 
  of 
  Teleosts 
  does 
  not 
  lead 
  to 
  the 
  for- 
  

   mation 
  of 
  a 
  continuous 
  plate 
  of 
  cells 
  underlying 
  the 
  sensory 
  layer 
  of 
  

   the 
  epiblast, 
  and 
  of 
  the 
  same 
  area 
  as 
  the 
  latter. 
  A 
  very 
  considerable 
  

   area 
  beneath 
  the 
  epiblast, 
  and 
  occupying 
  an 
  excentric 
  position 
  in 
  the 
  

   blastodisk, 
  is 
  not 
  invaded 
  by 
  the 
  ingrowing 
  hypoblastic 
  layer. 
  The 
  

   space 
  not 
  so 
  invaded 
  and 
  bounded 
  by 
  the 
  epiblastic 
  layer 
  above, 
  the 
  

   inflected 
  lips 
  of 
  the 
  hypoblastic 
  layer 
  at 
  the 
  sides, 
  and 
  by 
  the 
  periblast 
  

   below, 
  is 
  the 
  depressed 
  or 
  flattened 
  blastoccel 
  of 
  the 
  Teleostean 
  ovum. 
  

   It 
  is 
  perfectly 
  homologous 
  with 
  that 
  of 
  Branchiostoma, 
  but 
  is 
  flattened 
  

   or 
  depressed 
  by 
  the 
  way 
  in 
  which 
  the 
  growth 
  of 
  the 
  blastula 
  is 
  modified 
  

   by 
  the 
  presence 
  of 
  a 
  large 
  yelk, 
  which 
  is 
  itself 
  invested 
  by 
  the 
  peri- 
  

   blast 
  or 
  yelk-hypoblast. 
  The 
  blastula 
  becomes, 
  in 
  fact, 
  a 
  hollow 
  disk. 
  

  

  The 
  yelk-periblast 
  or 
  yelk-hypoblast, 
  and 
  the 
  inflected 
  hypoblast 
  are 
  

   hypoblastic 
  in 
  their 
  relations, 
  and 
  must 
  accordingly 
  be 
  invested 
  in 
  the 
  

   course 
  of 
  development 
  by 
  the 
  epiblast 
  by 
  epibole. 
  The 
  epiblast 
  and 
  

   hypoblast 
  are 
  accordingly 
  fused 
  at 
  the 
  margin 
  of 
  the 
  blastodisk. 
  The 
  

   entire 
  margin 
  of 
  the 
  blastodisk 
  must 
  consequently 
  be 
  regarded 
  as 
  the 
  

   blastopore 
  or 
  archistome 
  of 
  the 
  developing 
  Teleostean 
  ovum, 
  as 
  origi- 
  

   nally 
  implied 
  by 
  Haeckel. 
  Such 
  a 
  condition 
  of 
  things 
  would 
  be 
  brought 
  

   about 
  by 
  loading 
  or 
  surcharging 
  the 
  cells 
  of 
  the 
  hypoblastic 
  pole 
  of 
  the 
  

   blastula 
  of 
  Branchiostoma 
  with 
  yelk 
  substance. 
  The 
  way 
  in 
  which 
  the 
  

   discoblastula 
  arose 
  may 
  be 
  very 
  easily 
  understood, 
  if 
  the 
  gastrula 
  stages 
  

   of 
  BrancMostoma, 
  Rana, 
  and 
  Gadus 
  are 
  carefully 
  compared. 
  

  

  With 
  the 
  gradual 
  increase 
  in 
  the 
  size 
  of 
  the 
  yelk 
  in 
  the 
  vertebrate 
  

   series, 
  it 
  finally 
  happens 
  that 
  the 
  principal 
  morphol 
  ogical 
  features 
  of 
  

   the 
  embryo 
  are 
  distinctly 
  developed 
  long 
  before 
  yelk 
  absorption 
  is 
  com- 
  

   pleted. 
  As 
  a 
  result 
  of 
  this, 
  the 
  yelk, 
  which 
  occupies 
  a 
  ventral 
  position 
  

   in 
  reference 
  to 
  the 
  intestine, 
  is 
  finally 
  excluded 
  from 
  direct 
  connection 
  

   with 
  the 
  latter, 
  and 
  the 
  periblast 
  (hypoblastic 
  in 
  origin) 
  is 
  made 
  to 
  as- 
  

   sume 
  a 
  new 
  function, 
  viz, 
  that 
  of 
  ministering 
  to 
  the 
  incorporation 
  of 
  

  

  11 
  GoBtte. 
  Berlin, 
  medicin. 
  Centralblatt, 
  1869, 
  No. 
  26, 
  pp. 
  404-466, 
  and 
  Arch, 
  fttr 
  mik. 
  

   Anat., 
  ix, 
  1873, 
  p. 
  679. 
  

  

  12 
  Haeckel. 
  Die 
  Gastrula 
  und 
  die 
  Eifurckung. 
  Jena. 
  Zeitschr., 
  ix, 
  1875. 
  

  

  13 
  Henneguy. 
  Note 
  sur 
  quelques 
  faita 
  relatifs 
  aux 
  premiers 
  ph6nomenes 
  du 
  devel- 
  

   oppenient 
  des 
  poissous 
  osseux. 
  Bull. 
  Soc. 
  Philom. 
  de 
  Paris, 
  10 
  Apr., 
  1880, 
  p. 
  4. 
  

  

  "Kingsley 
  and 
  Conn. 
  Some 
  observations 
  on 
  the 
  embryology 
  of 
  Teleosts. 
  Mem. 
  

   Boat. 
  Soc. 
  Nat. 
  Hist., 
  iii, 
  pp. 
  183-212, 
  pis. 
  xiv-xvi, 
  1883. 
  

  

  