﻿[7] 
  DEVELOPMENT 
  OF 
  OSSEOUS 
  FISHES. 
  495 
  

  

  The 
  strenuous 
  opposition 
  to 
  the 
  doctrine 
  of 
  concrescence 
  manifested 
  

   by 
  Balfour 
  arose 
  apparently 
  from 
  his 
  too 
  constantly 
  interpreting 
  all 
  of 
  

   the 
  higher 
  vertebrate 
  types 
  of 
  development 
  upon 
  the 
  basis 
  of 
  his 
  bril- 
  

   liant 
  researches 
  on 
  the 
  Elasmobranchs. 
  Later 
  research 
  has 
  only 
  con- 
  

   firmed 
  the 
  doctrine 
  and 
  added 
  little 
  except 
  a 
  clearer 
  knowledge 
  of 
  the 
  

   details 
  of 
  the 
  process, 
  and 
  I 
  may 
  add 
  that 
  Whitman 
  has 
  recently 
  gone 
  

   over 
  the 
  subject 
  of 
  the 
  concrescence 
  of 
  the 
  germ 
  bands 
  in 
  Clepsine, 
  and 
  

   will 
  shortly 
  present 
  the 
  most 
  conclusive 
  evidence 
  of 
  the 
  soundness 
  of 
  

   the 
  views 
  which 
  he 
  originally 
  published 
  in 
  1878. 
  The 
  evidence 
  as 
  to 
  its 
  

   occurrence 
  in 
  Gadus 
  becomes 
  palpable 
  upon 
  the 
  advent 
  of 
  the 
  initial 
  

   steps 
  of 
  the 
  inflection 
  of 
  the 
  blastodermic 
  margin. 
  The 
  inflected 
  layer 
  

   soon 
  becomes 
  wider 
  in 
  the 
  region 
  wliere 
  the 
  embryo 
  is 
  formed 
  and 
  is 
  

   prolonged 
  with 
  the 
  progress 
  of 
  the 
  extension 
  of 
  the 
  blastoderm 
  over 
  the 
  

   yelk. 
  The 
  concrescence 
  of 
  the 
  lips 
  of 
  the 
  primitive 
  blastopore 
  in 
  the 
  

   middle 
  line 
  of 
  the 
  embryo 
  would 
  also 
  tend 
  to 
  carry 
  a 
  larger 
  number 
  of 
  

   periblastic 
  nuclei 
  under 
  the 
  anterior 
  and 
  middle 
  region 
  of 
  the 
  true 
  hypo- 
  

   blast. 
  It 
  does 
  not 
  necessarily 
  follow, 
  however, 
  that 
  the 
  floor 
  of 
  the 
  in- 
  

   testine 
  is 
  formed 
  by 
  cells 
  derived 
  from 
  the 
  periblast, 
  as 
  occurs 
  in 
  Elas- 
  

   mobranchs 
  (teste 
  Balfour), 
  and 
  as 
  held 
  by 
  Cunningham, 
  though 
  there 
  

   is 
  no 
  objection 
  to 
  such 
  a 
  view. 
  The 
  principal 
  conclusions 
  reached 
  by 
  

   Cunningham 
  as 
  to 
  the 
  homologies 
  of 
  the 
  yelk-blastopore 
  are, 
  however, 
  

   not 
  new, 
  as 
  Eauber 
  23 
  and 
  myself 
  24 
  had 
  previously 
  reached 
  the 
  same 
  or 
  

   very 
  similar 
  interpretations. 
  

  

  Eauber 
  considers 
  what 
  I 
  have 
  called 
  the 
  " 
  yelk-blastopore" 
  to 
  repre- 
  

   sent 
  the 
  " 
  blastostomion 
  verum 
  " 
  or 
  true 
  blastopore 
  in 
  types 
  with 
  a 
  large 
  

   yelk. 
  The 
  blastopore 
  of 
  the 
  latter 
  types, 
  usually 
  regarded 
  as 
  such 
  by 
  

   embryologists, 
  and 
  developed 
  at 
  the 
  anterior 
  part 
  of 
  the 
  primitive 
  streak, 
  

   Eauber 
  calls 
  the 
  " 
  blastostomion 
  consecutivum 
  seu 
  intermedium.' 
  11 
  Both 
  

   openings 
  together 
  are 
  for 
  him 
  simply 
  differentiations 
  of 
  the 
  primitive 
  

   " 
  blastostome." 
  This 
  blastostome, 
  or 
  the 
  blastopore, 
  as 
  it 
  is 
  usually 
  

   called, 
  of 
  the 
  Bilateralia, 
  whether 
  round 
  or 
  drawn 
  out 
  into 
  a 
  cleft 
  by 
  a 
  

   process 
  of 
  growth 
  in 
  length, 
  or 
  of 
  concrescence, 
  I 
  have 
  elsewhere 
  25 
  dis- 
  

   tinguished 
  as 
  the 
  archistome. 
  

  

  (2) 
  Later 
  development. 
  — 
  The 
  more 
  advanced 
  stages 
  of 
  the 
  cod 
  embryo, 
  

   though 
  studied 
  by 
  Sars 
  and 
  others, 
  have 
  never 
  been 
  correctly 
  figured 
  

   by 
  any 
  one 
  because 
  of 
  the 
  fact 
  that 
  a 
  very 
  large 
  vesicle 
  on 
  the 
  upper 
  

   side 
  of 
  the 
  head 
  has 
  been 
  entirely 
  overlooked, 
  probably 
  because 
  of 
  the 
  

   extreme 
  transparency 
  of 
  this 
  portion 
  of 
  the 
  young 
  fish. 
  In 
  my 
  first 
  

   paper 
  on 
  the 
  development 
  of 
  the 
  cod 
  (Contrib. 
  Embryog. 
  Oss. 
  Fishes, 
  

   pis. 
  xi 
  and 
  xii), 
  figs. 
  45 
  and 
  49, 
  which 
  are 
  side 
  views 
  of 
  embryos, 
  

   figured 
  ten 
  and 
  seven 
  days 
  after 
  hatching, 
  give 
  the 
  erroneous 
  impres- 
  

   sion 
  that 
  the 
  median 
  fin-fold 
  extends 
  quite 
  forward 
  between 
  the 
  nasal 
  

  

  23 
  Eauber. 
  Die 
  Lage 
  der 
  Keimpforte. 
  Zoolog. 
  Anzeiger, 
  ii, 
  1879, 
  pp. 
  499-503. 
  

   24 
  Kyder. 
  On 
  the 
  position 
  of 
  the 
  yelk-blastopore 
  as 
  determined 
  by 
  the 
  size 
  of 
  the 
  

   vitellus. 
  Am. 
  Naturalist, 
  April, 
  1885, 
  pp. 
  411-415. 
  

  

  25 
  Eyder. 
  The 
  Arcbiatome 
  Theory. 
  Am. 
  Naturalist, 
  Nov., 
  1885, 
  pp. 
  1115-1121. 
  

  

  