﻿I 
  33] 
  DEVELOPMENT 
  OF 
  OSSEOUS 
  FISHES. 
  521 
  

  

  bladder 
  al 
  and 
  the 
  pronephric 
  funnel 
  pn 
  anteriorly. 
  The 
  chorda 
  cli 
  is 
  

   cut 
  through 
  at 
  several 
  points, 
  and 
  the 
  iioor 
  of 
  the 
  pericardial 
  cavity 
  pc 
  

   is 
  shown 
  to 
  be 
  very 
  thin 
  and 
  continuous 
  with 
  the 
  venous 
  end 
  of 
  the 
  

   heart 
  h. 
  The 
  latter, 
  therefore, 
  opens 
  directly 
  into 
  the 
  body 
  cavity 
  be- 
  

   hind 
  it, 
  the 
  body 
  cavity 
  itself 
  being 
  directly 
  continuous 
  with 
  the 
  

   cleavage 
  cavity 
  of 
  an 
  earlier 
  stage, 
  the 
  latter 
  becoming 
  the 
  former 
  in 
  the 
  

   course 
  of 
  development. 
  These 
  sections 
  were 
  prepared 
  from 
  Spanish 
  

   mackerel 
  embryos 
  which 
  had 
  left 
  the 
  egg 
  only 
  about 
  twenty-four 
  hours, 
  

   and 
  measured 
  about 
  3 
  lum 
  in 
  length. 
  

  

  XV. 
  — 
  Ch^etodipterus 
  faber 
  (Brouss.) 
  J. 
  & 
  G. 
  {Angel-Fish; 
  

  

  For 
  gee, 
  or 
  Moorish.) 
  

  

  The 
  ova 
  of 
  this 
  species 
  are 
  pelagic 
  or 
  floating 
  in 
  habit, 
  iu 
  sea-water 
  

   having 
  a 
  specific 
  gravity 
  of 
  1.014. 
  They 
  hatch 
  out 
  in 
  twenty-four 
  hours 
  

   when 
  the 
  temperature 
  of 
  the 
  water 
  is 
  80° 
  Fahr. 
  This 
  species 
  spawns 
  in 
  

   the 
  Chesapeake 
  during 
  the 
  latter 
  part 
  of 
  June 
  and 
  the 
  early 
  part 
  of 
  

   July. 
  It 
  is 
  prodigiously 
  fertile, 
  the 
  female 
  probably 
  discharging 
  a 
  

   million 
  ova 
  during 
  a 
  single 
  season. 
  

  

  The 
  egg 
  measures 
  somewhat 
  over 
  a 
  millimeter 
  in 
  diameter. 
  The 
  

   blastodisk 
  will 
  develop 
  independently 
  of 
  fertilization, 
  as 
  shown 
  in 
  fig. 
  

   57. 
  Cleavage 
  is 
  very 
  rapid 
  and 
  it 
  requires 
  only 
  about 
  one 
  hour 
  to 
  pass 
  

   from 
  the 
  condition 
  shown 
  in 
  fig. 
  58 
  to 
  the 
  morula 
  condition 
  shown 
  in 
  

   fig. 
  GO. 
  Fig. 
  59 
  shows 
  the 
  form 
  assumed 
  by 
  the 
  cells 
  of 
  the 
  morula 
  

   stage, 
  and 
  traces 
  are 
  also 
  present 
  of 
  the 
  subquadrate 
  form 
  assumed 
  by 
  

   the 
  blastodisk 
  at 
  the 
  time 
  of 
  the 
  completion 
  of 
  the 
  second 
  cleavage. 
  

  

  After 
  the 
  blastodisk 
  begins 
  to 
  spread, 
  the 
  development 
  of 
  the 
  mar- 
  

   ginal 
  cells 
  at 
  its 
  edge 
  becomes 
  very 
  well 
  marked, 
  as 
  shown 
  in 
  fig. 
  01. 
  

   The 
  oil-drop, 
  shown 
  in 
  figs. 
  58, 
  GO, 
  and 
  Gl, 
  remains 
  for 
  a 
  time 
  almost 
  

   exactly 
  opposite 
  the 
  center 
  of 
  the 
  blastodisk, 
  and 
  later, 
  when 
  the 
  em- 
  

   bryo 
  is 
  formed, 
  it 
  occupies 
  a 
  median 
  ventral 
  position 
  in 
  the 
  yelk-sack, 
  

   as 
  shown 
  in 
  figs. 
  62 
  and 
  63, 
  and 
  is 
  not 
  finally 
  shoved 
  to 
  the 
  posterior 
  

   end 
  of 
  the 
  latter, 
  as 
  in 
  the 
  embryo 
  Spanish 
  mackerel. 
  

  

  In 
  thirteen 
  hours 
  the 
  embryo 
  fish 
  is 
  fairly 
  outlined, 
  as 
  shown 
  in 
  fig. 
  

   62, 
  and 
  the 
  oil-drop 
  becomes 
  covered 
  by 
  the 
  periblast 
  in 
  which 
  nuclei 
  

   seem 
  to 
  be 
  differentiated. 
  At 
  this 
  stage 
  faintly-colored 
  pigment 
  cells, 
  

   mostly 
  of 
  a 
  rounded 
  form, 
  become 
  developed 
  on 
  the 
  body 
  of 
  the 
  embryo. 
  

   At 
  the 
  under 
  side 
  of 
  the 
  tail 
  Kupffer's 
  vesicle 
  is 
  also 
  distinctly 
  developed. 
  

   All 
  over 
  the 
  blastoderm, 
  enveloping 
  the 
  yelk 
  at 
  this 
  stage, 
  faintly-colored 
  

   pigment 
  cells 
  are 
  discernible. 
  

  

  At 
  the 
  end 
  of 
  twenty-four 
  hours 
  the 
  young 
  fish 
  leave 
  the 
  egg, 
  measur- 
  

   ing 
  about 
  2.5 
  mm 
  in 
  length. 
  Sixteen 
  hours 
  later, 
  as 
  shown 
  in 
  fig. 
  63, 
  

   the 
  embryo 
  has 
  grown 
  about 
  half 
  a 
  millimeter 
  in 
  length, 
  and 
  the 
  

   greater 
  part 
  of 
  the 
  yelk 
  has 
  been 
  absorbed, 
  so 
  that 
  an 
  immense 
  se- 
  

   rous 
  space 
  is 
  left 
  in 
  front 
  of 
  the 
  yelk, 
  between 
  the 
  periblast 
  envelop- 
  

   ing 
  the 
  latter 
  and 
  the 
  outer 
  somatopleural 
  sack. 
  This 
  wide 
  space 
  has 
  

  

  