﻿63 
  

  

  the 
  vitelliis, 
  but 
  inerely 
  the 
  protrusion 
  of 
  one 
  side 
  into 
  a 
  

   more 
  or 
  less 
  prominent 
  liml). 
  Fig. 
  a, 
  Pi. 
  I. 
  In 
  certain 
  

   ova, 
  however, 
  glomeration 
  did 
  take 
  place: 
  that 
  is, 
  round 
  

   balls, 
  sometimes 
  one 
  above 
  the 
  other, 
  formed 
  irregularly 
  

   over 
  the 
  blastodermic 
  tissue, 
  and 
  I 
  noticed 
  that 
  such 
  

   formations, 
  in 
  both 
  Smelt 
  and 
  Shad 
  ova, 
  heralded 
  always 
  

   the 
  disintegration 
  of 
  the 
  vitellus. 
  Sometimes 
  this 
  glomera- 
  

   tion 
  took 
  place 
  after 
  the 
  embryo 
  was 
  considerably 
  formed. 
  

   in 
  all 
  cases, 
  however, 
  no 
  further 
  development 
  took 
  j)lace, 
  

   the 
  ovum 
  soon 
  becoming 
  opaque 
  and 
  dead. 
  Coincident, 
  or 
  

   nearly 
  so_, 
  with 
  the 
  extension 
  of 
  the 
  blastoderm 
  is 
  the 
  forma- 
  

   tion 
  from 
  the 
  blastoderm 
  over 
  tlie 
  entire 
  surface 
  of 
  the 
  vitel- 
  

   lus, 
  of 
  a 
  layer 
  of 
  cclls_, 
  the 
  exoderm, 
  (epiblast),* 
  which 
  forms 
  

   the 
  outer 
  c^at 
  or 
  skin 
  of 
  the 
  embryo. 
  

  

  The 
  folding 
  which 
  this 
  layer 
  undergoes 
  along 
  what 
  is 
  (o 
  

   be 
  the 
  dorsum 
  of 
  the 
  embryo, 
  I 
  was 
  unable 
  to 
  make 
  out 
  sat- 
  

   isfactorily, 
  on 
  account 
  of 
  the 
  difficulty 
  of 
  keeping 
  the 
  blasto- 
  

   derm 
  in 
  a 
  proper 
  position 
  for 
  study 
  under 
  the 
  microscope. 
  

   Almost 
  as 
  soon 
  as 
  it 
  was 
  got 
  into 
  a 
  good 
  position, 
  it 
  ATould 
  

   roll 
  slowly 
  back 
  to 
  the 
  side. 
  I 
  thought 
  I 
  made 
  out, 
  however^ 
  

   in 
  a 
  number 
  of 
  si^ccimens, 
  the 
  folding 
  in 
  different 
  stages 
  of 
  

   its 
  progress. 
  

  

  This 
  folding 
  consists 
  of 
  the 
  growing 
  up 
  along 
  the 
  central 
  

   portion 
  of 
  the 
  blastoderm 
  of 
  two 
  laminae 
  or 
  ridges 
  from 
  the 
  

   exoderm. 
  These 
  lamina; 
  finally 
  unite 
  along 
  a 
  median 
  line, 
  

   leaving 
  a 
  central 
  cavity 
  or 
  tube 
  the 
  length 
  of 
  the 
  folding, 
  

  

  *I 
  use 
  the 
  terms 
  cvodi'mi 
  and 
  endodenn 
  as 
  being 
  more 
  exact 
  in 
  meaning 
  

   than 
  epiblast 
  and 
  hypoblast. 
  The 
  exodermic 
  layer 
  is 
  ■\vilh 
  fishes, 
  very 
  rarely 
  

   fwrmed 
  upon 
  tlie 
  upper 
  side 
  of 
  the 
  blastoderm, 
  yet, 
  according 
  to 
  the 
  meaning 
  

   of 
  the 
  terms, 
  when 
  the 
  epiblast 
  is 
  formed 
  below, 
  or 
  on 
  the 
  lower 
  side 
  of 
  the 
  

   hlafrtoderm, 
  it 
  would 
  ])ecome 
  the 
  hypoblast. 
  In 
  the 
  same 
  manner 
  the 
  hypo- 
  

   l)]ast 
  or 
  under 
  blast, 
  when 
  formed 
  above 
  the 
  blastoderm, 
  would 
  become 
  the 
  

   epiblast 
  I 
  use 
  these 
  terms 
  also 
  because 
  they 
  are 
  used 
  for 
  similar 
  or 
  homol- 
  

   ogous 
  structures 
  among 
  the 
  lower 
  forms 
  of 
  animal 
  life. 
  And 
  similarity 
  is 
  

   much 
  to 
  be 
  desired 
  in 
  names 
  bearing 
  upon 
  or 
  attached 
  to 
  homologous 
  strup- 
  

   lurt'8. 
  

  

  