﻿498 
  REPORT 
  OF 
  COMMISSIONER 
  OF 
  FISH 
  AND 
  FISHERIES. 
  [10] 
  

  

  In 
  larvae 
  of 
  Gadus 
  hardened 
  in 
  chromic 
  acid 
  there 
  seems 
  to 
  be 
  such 
  a 
  

   coagulum 
  existing 
  in 
  the 
  dorsal 
  bulla 
  already 
  fully 
  described. 
  This 
  

   bulla 
  therefore 
  partakes 
  of 
  the 
  nature 
  of 
  a 
  lymph-space. 
  

  

  (3) 
  Changes 
  of 
  position 
  of 
  the 
  cod's 
  egg 
  and 
  embryo 
  during 
  development. 
  — 
  

   The 
  germinal 
  disk 
  of 
  the 
  cod's 
  egg, 
  like 
  that 
  of 
  the 
  ova 
  of 
  most 
  Teleos- 
  

   teans, 
  is 
  developed 
  at 
  the 
  time 
  of 
  impregnation. 
  The 
  single 
  spermato- 
  

   zoon 
  necessary 
  to 
  effect 
  impregnation 
  and 
  initiate 
  developement 
  enters 
  

   the 
  egg 
  through 
  a 
  minute 
  round 
  pore 
  in 
  the 
  egg-membrane 
  or 
  zona 
  

   radiata, 
  known 
  as 
  the 
  micropyle. 
  But 
  one 
  such 
  opening 
  is 
  found 
  in 
  the 
  

   egg 
  of 
  the 
  cod, 
  and 
  I 
  believe 
  that 
  reliable 
  authorities 
  concur 
  in 
  the 
  be- 
  

   lief 
  that 
  there 
  is 
  but 
  one 
  such 
  opening 
  in 
  the 
  membrane 
  which 
  invests 
  

   the 
  ovum 
  of 
  Teleosts. 
  

  

  The 
  male 
  element 
  can 
  therefore 
  enter 
  the 
  ovum 
  at 
  one 
  point 
  only, 
  

   and, 
  inasmuch 
  as 
  the 
  superficial 
  cortical 
  layer 
  at 
  the 
  time 
  of 
  impregna- 
  

   tion, 
  and 
  from 
  which 
  the 
  blastodisk 
  or 
  germ 
  is 
  developed, 
  lies 
  in 
  imme- 
  

   diate 
  contact 
  with 
  the 
  egg-membrane 
  or 
  zona, 
  the 
  point 
  of 
  contact 
  be 
  

   tween 
  the 
  egg 
  and 
  spermatozoon 
  is 
  also 
  limited 
  by 
  the 
  area 
  on 
  the 
  ovum 
  

   covered 
  by 
  the 
  micropyle. 
  That 
  area 
  is 
  excessively 
  small. 
  The 
  polar 
  

   cells 
  in 
  the 
  cod's 
  egg 
  are 
  also 
  extruded 
  immediately 
  beneath 
  the 
  micro- 
  

   pyle 
  and 
  invariably 
  in 
  very 
  close 
  relation 
  to 
  it. 
  Furthermore, 
  the 
  active 
  

   plasma 
  of 
  the 
  egg 
  gravitates 
  towards 
  the 
  point 
  where 
  the 
  spermatozoon 
  

   entered 
  the 
  egg, 
  and 
  the 
  greater 
  part 
  thus 
  accumulates 
  in 
  the 
  vicinity 
  

   of 
  the 
  micropyle, 
  as 
  shown 
  in 
  fig. 
  3, 
  pi. 
  i, 
  where 
  the 
  polar 
  cells 
  joining 
  

   the 
  egg 
  to 
  its 
  membrane 
  are 
  also 
  indicated. 
  I 
  never 
  saw 
  any 
  polar 
  

   globules 
  expelled 
  through 
  the 
  micropyle. 
  * 
  

  

  As 
  soon 
  as 
  the 
  blastodisk 
  becomes 
  apparent 
  as 
  a 
  thickening 
  or 
  ag- 
  

   gregation 
  of 
  the 
  substance 
  of 
  the 
  cortical 
  layer 
  it 
  assumes 
  an 
  inferior 
  

   position, 
  because 
  the 
  specific 
  gravity 
  of 
  the 
  plasma 
  of 
  the 
  disk 
  is 
  greater 
  

   than 
  the 
  same 
  volume 
  of 
  yelk, 
  the 
  whole 
  of 
  which 
  now 
  occupies 
  the 
  

   upper 
  pole 
  of 
  the 
  egg. 
  Later 
  still, 
  when 
  the 
  disk 
  D, 
  fig. 
  4, 
  is 
  better 
  

   defined, 
  the 
  force 
  of 
  gravity, 
  still 
  acting 
  in 
  the 
  direction 
  of 
  the 
  arrow, 
  

   which 
  points 
  toward 
  the 
  micropyle, 
  constantly 
  keeps 
  the 
  disk 
  in 
  an 
  in- 
  

   ferior 
  position, 
  which 
  is 
  maintained 
  until 
  the 
  blastodisk 
  begins 
  to 
  

   spread 
  and 
  the 
  embryo 
  to 
  be 
  formed. 
  When 
  the 
  blastodisk 
  or 
  the 
  

   blastoderm, 
  as 
  it 
  may 
  now 
  be 
  more 
  appropriately 
  called, 
  has 
  spread 
  

   over 
  one-half 
  of 
  the 
  vitelline 
  globe, 
  as 
  shown 
  in 
  fig. 
  5, 
  the 
  embryo 
  is 
  

   pretty 
  well 
  defined 
  at 
  one 
  side 
  of 
  the 
  blastoderm, 
  and 
  extends 
  from 
  its 
  

   margin 
  to 
  its 
  center. 
  This 
  causes 
  the 
  blastoderm 
  to 
  become 
  heavier 
  

   at 
  the 
  side 
  upon 
  which 
  the 
  embryo 
  is 
  formed, 
  and, 
  as 
  a 
  consequence, 
  

   the 
  whole 
  egg 
  is 
  slightly 
  rotated 
  upon 
  its 
  own 
  center 
  so 
  that 
  a 
  radius 
  

   drawn 
  from 
  the 
  latter 
  to 
  the 
  center 
  of 
  the 
  original 
  site 
  of 
  the 
  blastodisk 
  

   of 
  an 
  earlier 
  stage 
  will 
  be 
  inclined 
  to 
  the 
  horizon 
  at 
  an 
  angle 
  of 
  45°. 
  

   &.s 
  development 
  proceeds 
  still 
  further 
  the 
  embryo 
  of 
  course 
  lengthens 
  

   as 
  the 
  blastoderm 
  spreads, 
  till 
  finally 
  the 
  embryo 
  embraces 
  an 
  arc 
  of 
  

   180° 
  on 
  the 
  yelk-globe. 
  As 
  a 
  result 
  of 
  this 
  the 
  radius 
  passing 
  from 
  

   the 
  center 
  of 
  the 
  egg 
  to 
  the 
  original 
  site 
  of 
  the 
  center 
  of 
  the 
  blastodisk 
  

  

  