﻿512 
  REPORT 
  OF 
  COMMISSIONER 
  OF 
  FISH 
  AND 
  FISHERIES. 
  [24] 
  

  

  There 
  is 
  no 
  germinal 
  disk 
  developed 
  when 
  the 
  egg 
  first 
  leaves 
  the 
  

   ovary, 
  and 
  the 
  cortical 
  layer 
  of 
  germinal 
  matter 
  is 
  uniformly 
  distributed 
  

   at 
  first 
  over 
  the 
  vitellus, 
  which 
  itself 
  incloses 
  a 
  number 
  of 
  very 
  re- 
  

   fringeut 
  oil 
  spheres, 
  very 
  variable 
  in 
  size. 
  It 
  appears 
  that 
  the 
  blasto- 
  

   disk 
  in 
  this 
  species 
  may 
  develop 
  without 
  the 
  influence 
  of 
  impregna- 
  

   tion, 
  but 
  no 
  true 
  segmentation 
  occurs 
  under 
  such 
  circumstances. 
  

  

  Un 
  the 
  fourth 
  or 
  fifth 
  day 
  after 
  impregnation 
  the 
  primary 
  divisions 
  

   of 
  the 
  brain 
  are 
  marked 
  off, 
  one 
  of 
  the 
  most 
  striking 
  characters 
  being 
  

   the 
  unusual 
  spaciousness 
  of 
  the 
  cerebral 
  vesicles, 
  the 
  walls 
  of 
  the 
  brain 
  

   cavity 
  being 
  relatively 
  thin 
  when 
  compared 
  with 
  those 
  of 
  other 
  forms. 
  

   The 
  optic 
  cups 
  soon 
  become 
  quite 
  deep, 
  so 
  that 
  a 
  considerable 
  space 
  

   (the 
  vitreous 
  humor) 
  exists 
  at 
  an 
  early 
  period 
  between 
  the 
  floor 
  of 
  the 
  

   cup 
  and 
  the 
  lens. 
  The 
  origin 
  of 
  the 
  latter 
  from 
  a 
  thickened 
  induplica- 
  

   tion 
  of 
  the 
  epiblast 
  may 
  be 
  very 
  readily 
  traced. 
  Some 
  of 
  these 
  and 
  

   other 
  features 
  to 
  be 
  described 
  later, 
  are 
  represented 
  in 
  figs. 
  22, 
  23, 
  and 
  

   24, 
  plate 
  v. 
  Immediately 
  behind 
  the 
  auditory 
  vesicles, 
  and 
  shortly 
  

   after 
  their 
  invagination, 
  the 
  rudiments 
  of 
  the 
  breast 
  fins 
  appear 
  as 
  a 
  

   pair 
  of 
  low 
  longitudinal 
  folds. 
  In 
  the 
  stickleback 
  the 
  breast 
  or 
  pectoral 
  

   tins 
  develop 
  very 
  rapidly 
  and 
  while 
  the 
  young 
  fish 
  is 
  still 
  in 
  the 
  egg. 
  

  

  Pigment 
  is 
  also 
  rapidly 
  developed 
  on 
  the 
  embryo, 
  as 
  is 
  shown 
  in 
  figs. 
  

   25 
  and 
  26, 
  plate 
  v, 
  representing 
  the 
  young, 
  6 
  mm 
  long, 
  of 
  Apeltes 
  when 
  

   it 
  quits 
  the 
  egg. 
  During 
  still 
  earlier 
  stages 
  and 
  while 
  still 
  in 
  the 
  egg 
  

   pigment 
  is 
  formed 
  so 
  rapidly 
  over 
  the 
  embryo 
  that 
  it 
  soon 
  becomes 
  im- 
  

   possible 
  to 
  see 
  the 
  outlines 
  of 
  the 
  viscera 
  through 
  the 
  mantle 
  of 
  crowded 
  

   pigment 
  cells 
  ; 
  such 
  is 
  the 
  case 
  with 
  a 
  still 
  older 
  stage 
  represented 
  by 
  

   fig. 
  27, 
  plate 
  vi. 
  About 
  the 
  time 
  of 
  hatching, 
  a 
  second 
  kind 
  of 
  pig- 
  

   ment 
  cells, 
  brown 
  in 
  color 
  instead 
  of 
  black, 
  and 
  much 
  larger 
  than 
  the 
  

   latter, 
  make 
  their 
  appearance. 
  These 
  brown 
  pigment 
  cells 
  blotch 
  the 
  

   embryo 
  symmetrically 
  on 
  the 
  sides 
  and 
  along 
  the 
  median 
  dorsal 
  line, 
  

   being 
  confined 
  to 
  sharply 
  circumscribed 
  areas 
  in 
  those 
  regions, 
  as 
  may 
  

   be 
  gathered 
  from 
  figs. 
  25 
  and 
  26. 
  The 
  style 
  of 
  pigmentation 
  preva- 
  

   lent 
  at 
  the 
  time 
  of 
  hatching 
  foreshadows 
  that 
  of 
  the 
  adult. 
  

  

  The 
  heart 
  appears 
  about 
  the 
  fourth 
  day 
  as 
  a 
  heap 
  of 
  mesoblastic 
  cells 
  

   just 
  below 
  the 
  hinder 
  part 
  of 
  the 
  head, 
  and 
  is 
  at 
  first 
  a 
  simple 
  sinus. 
  

   It 
  does 
  not 
  begin 
  to 
  pulsate 
  vigorously 
  until 
  the 
  seventh 
  day, 
  when 
  its 
  

   pulsations 
  are 
  nearly 
  if 
  not 
  quite 
  100 
  per 
  minute. 
  Its 
  venous 
  end 
  rap- 
  

   idly 
  elongates 
  until 
  it 
  extends 
  fully 
  the 
  diameter 
  of 
  the 
  body 
  beyond 
  

   the 
  right 
  side 
  of 
  the 
  embryo; 
  a 
  large 
  pericardial 
  space 
  is 
  developed 
  be- 
  

   low 
  the 
  head 
  at 
  this 
  point 
  for 
  its 
  lodgment; 
  this 
  space 
  dips 
  down 
  deeply 
  

   into 
  the 
  amber-colored 
  vitellus. 
  It 
  continues 
  to 
  pulsate 
  from 
  this 
  time 
  

   onwards, 
  but 
  there 
  are 
  as 
  yet 
  no 
  blood 
  corpuscles. 
  A 
  wide 
  space 
  now 
  

   appears 
  on 
  the 
  right 
  side 
  of 
  the 
  embryo 
  and 
  underneath 
  the 
  latter. 
  

   This 
  latter 
  we 
  may 
  consider 
  as 
  a 
  vascular 
  sinus 
  or 
  channel 
  of 
  definite 
  

   outline. 
  The 
  floor 
  of 
  this 
  space, 
  as 
  far 
  as 
  I 
  have 
  been 
  able 
  to 
  observe, 
  

   seems 
  to 
  consist 
  of 
  the 
  periblast 
  (hypoblast), 
  from 
  which 
  knobbed 
  cells 
  

   project 
  upward, 
  and 
  which 
  appear 
  to 
  be 
  budding 
  off 
  portions 
  of 
  them- 
  

  

  