﻿[39] 
  DEVELOPMENT 
  OF 
  OSSEOUS 
  FISHES. 
  527 
  

  

  latter, 
  e, 
  from 
  the 
  epithelial 
  and 
  immediately 
  underlying 
  cells 
  of 
  the 
  

   disk. 
  From 
  these 
  two 
  primitive 
  organs 
  all 
  those 
  of 
  the 
  young 
  shad 
  

   are 
  evolved 
  by 
  further 
  differentiation, 
  folding, 
  expansion, 
  invagination, 
  

   &c. 
  

  

  The 
  central 
  nervous 
  system 
  is 
  developed 
  wholly 
  from 
  the 
  epiblast. 
  

   This 
  will 
  be 
  rendered 
  the 
  more 
  obvfous 
  from 
  a 
  glance 
  at 
  figs. 
  lOGtolOS, 
  

   representing 
  transverse 
  sections 
  through 
  the 
  embryonic 
  axis 
  of 
  an 
  em- 
  

   bryo 
  shad 
  represented 
  in 
  fig. 
  101, 
  plate 
  xvi. 
  The 
  spinal 
  cord 
  N 
  iu 
  

   these 
  is 
  shown 
  to 
  arise 
  as 
  a 
  thickening 
  of 
  the 
  layer 
  e, 
  and 
  extends 
  the 
  

   entire 
  length 
  of 
  the 
  body 
  at 
  this 
  stage, 
  and 
  is 
  here 
  characterized 
  by 
  its 
  

   solidity, 
  as 
  in 
  the 
  embryos 
  of 
  all 
  Osseous 
  fishes 
  and 
  the 
  Lampreys. 
  

   During 
  a 
  later 
  stage 
  the* 
  cord 
  N 
  becomes 
  separated 
  from 
  the 
  layer 
  e, 
  as 
  

   is 
  shown 
  iu 
  cross- 
  sections 
  (figs. 
  109 
  to 
  112) 
  of 
  a 
  much 
  later 
  stage, 
  viz, 
  

   that 
  shown 
  in 
  fig. 
  12G, 
  though 
  the 
  sundering 
  of 
  the 
  embryonic 
  spinal 
  

   cord 
  from 
  the 
  epiblast 
  in 
  reality 
  occurs 
  considerably 
  earlier, 
  or 
  by 
  the 
  

   time 
  the 
  stage 
  represented 
  iu 
  fig. 
  103 
  is 
  reached. 
  

  

  As 
  the 
  blastoderm 
  spreads, 
  the 
  portion 
  e, 
  fig. 
  140, 
  lengthens 
  to 
  form 
  

   the 
  embryo, 
  the 
  component 
  layers 
  of 
  the 
  latter, 
  fig. 
  9G, 
  e 
  and 
  h, 
  be- 
  

   coming 
  much 
  tbickened 
  in 
  the 
  vicinity 
  of 
  the 
  median 
  longitudinal 
  

   plane. 
  From 
  the 
  inferior 
  thickened 
  part 
  of 
  the 
  inflected 
  band 
  of 
  tissue 
  

   //, 
  tlie 
  chorda 
  ch, 
  figs. 
  100, 
  107, 
  and 
  108, 
  is 
  formed, 
  together 
  with 
  the 
  myo- 
  

   blasts 
  or 
  myotomes 
  on 
  either 
  side 
  of 
  the 
  chorda, 
  as 
  dorsal 
  outgrowths, 
  

   fig. 
  97, 
  M 
  M 
  M, 
  of 
  the 
  primitive 
  hypoblast 
  h. 
  * 
  These 
  rudiments 
  of 
  the 
  

   musculature 
  of 
  the 
  body 
  in 
  the 
  embryo 
  shad 
  are 
  not 
  hollow, 
  as 
  in 
  Bran- 
  

   chiostoma, 
  or 
  composed 
  of 
  two 
  layers 
  forming 
  the 
  inner 
  and 
  outer 
  walls 
  

   of 
  the 
  series 
  of 
  myoblastic 
  segments, 
  as 
  in 
  HJlasmobranchii. 
  They 
  are 
  

   therefore 
  not 
  clearly 
  defined 
  paired 
  archenteric 
  diverticula 
  or 
  gut 
  

   pouches, 
  but 
  their 
  solid 
  condition 
  is 
  probably 
  due 
  to 
  an 
  abbreviation 
  of 
  

   development 
  like 
  that 
  which 
  has 
  affected 
  and 
  retarded 
  the 
  appearance 
  

   of 
  the 
  cavity 
  of 
  the 
  spinal 
  cord 
  and 
  brain 
  and 
  that 
  of 
  the 
  intestine. 
  

   The 
  inferior 
  part 
  of 
  the 
  layer 
  h, 
  in 
  fig. 
  96, 
  gives 
  rise 
  in 
  the 
  middle 
  line 
  to 
  

   the 
  intestine 
  i, 
  as 
  shown 
  in 
  figs. 
  10G 
  and 
  107. 
  The 
  lumen 
  of 
  the 
  intes- 
  

   tinal 
  canal 
  appears 
  very 
  slowly, 
  and 
  at 
  first 
  is 
  a 
  mere 
  pore 
  in 
  cross- 
  

   sections, 
  as 
  shown 
  in 
  figs. 
  109 
  to 
  112 
  at 
  i. 
  During 
  the 
  early 
  stages 
  the 
  

   hypoblastic 
  baud 
  which 
  gives 
  rise 
  to 
  the 
  intestine 
  is 
  thickest 
  at 
  the 
  

   posterior 
  end 
  of 
  the 
  embryo, 
  and 
  gradually 
  thins 
  out 
  and 
  widens 
  as 
  it 
  

   extends 
  toward 
  the 
  head. 
  This 
  is 
  rendered 
  obvious 
  upon 
  comparing 
  a 
  

   section 
  near 
  the 
  tail 
  end 
  of 
  the 
  embryo, 
  fig. 
  Ill, 
  with 
  a 
  more 
  anterior 
  

   one, 
  fig. 
  109, 
  from 
  the 
  region 
  of 
  the 
  back 
  part 
  of 
  the 
  medulla 
  oblongata. 
  

   The 
  lumen 
  of 
  the 
  intestine 
  in 
  this 
  anterior 
  region 
  is 
  also 
  no 
  longer 
  pore- 
  

   like, 
  but 
  flattened, 
  cleft-like, 
  and 
  transverse. 
  

  

  At 
  a 
  slightly 
  earlier 
  stage 
  than 
  is 
  shown 
  by 
  the 
  sections 
  represented 
  

   in 
  figs. 
  100 
  and 
  107, 
  the 
  chorda 
  ch 
  is 
  found 
  to 
  be 
  united 
  interiorly 
  with 
  

   the 
  layer 
  i, 
  which 
  gives 
  rise 
  to 
  the 
  intestine. 
  The 
  chorda, 
  myotomes, 
  

   and 
  the 
  intestine 
  are 
  therefore 
  to 
  be 
  regarded 
  as 
  differentiations 
  of 
  the 
  

   layer 
  h 
  in 
  figs. 
  9G 
  and 
  118. 
  

  

  