﻿[5] 
  DEVELOPMENT 
  OF 
  OSSEOUS 
  FISHES. 
  493 
  

  

  the 
  vitelline 
  matter. 
  Kolhnann 
  15 
  has 
  lately 
  presented 
  some 
  strong 
  evi- 
  

   dence 
  in 
  favor 
  of 
  this 
  view, 
  but 
  is 
  in 
  error 
  in 
  assuming 
  that 
  the 
  lips 
  of 
  

   the 
  inflected 
  rim 
  (hypoblastic 
  stratum) 
  of 
  the 
  blastoderm 
  are 
  the 
  lips 
  

   of 
  the 
  blastopore. 
  If 
  this 
  view 
  is 
  admitted, 
  we 
  are 
  logically 
  forced 
  to 
  

   conclude 
  that 
  the 
  yelk 
  is 
  something 
  extraneous, 
  and 
  is 
  not 
  an 
  integral 
  

   part 
  of 
  the 
  ovum, 
  as 
  we 
  know 
  it 
  to 
  be, 
  judging 
  from 
  the 
  way 
  in 
  which 
  

   the 
  periblast 
  arises. 
  The 
  manner 
  in 
  which 
  the 
  latter 
  is 
  formed 
  shows 
  

   that 
  the 
  continuity 
  of 
  the 
  blastodisk 
  with 
  the 
  plasmic 
  layer 
  investing 
  

   the 
  yelk 
  is 
  perfect, 
  and 
  that 
  the 
  cleavage 
  cavity 
  is 
  exactly 
  homologous 
  

   with 
  that 
  developed 
  in 
  a 
  holoblastic 
  ovum. 
  There 
  is, 
  therefore, 
  a 
  blas- 
  

   tula 
  stage 
  developed 
  in 
  Teleosts 
  which 
  is 
  most 
  distinctly 
  evident 
  at 
  the 
  

   time 
  the 
  " 
  nuclear 
  zone" 
  is 
  formed. 
  The 
  vesicular 
  syncytium 
  formed 
  by 
  

   the 
  periblast 
  is, 
  however, 
  so 
  enormously 
  distended 
  with 
  passive 
  yelk 
  

   that 
  gastrulation 
  is 
  modified 
  to 
  an 
  extreme 
  degree. 
  Kollmann's 
  argu- 
  

   ments 
  against 
  the 
  final 
  closure 
  of 
  the 
  blastopore 
  at 
  the 
  edge 
  of 
  the 
  

   blastoderm 
  and 
  at 
  the 
  hinder 
  extremity 
  of 
  the 
  axis 
  of 
  the 
  embryo 
  breaks 
  

   down 
  completely, 
  if 
  the 
  processes 
  of 
  gastrulation 
  of 
  Branchiostoma, 
  Bana, 
  

   and 
  Gadus 
  are 
  compared, 
  because 
  such 
  a 
  comparison 
  shows 
  : 
  First, 
  that 
  

   a 
  gradual 
  loading 
  of 
  the 
  entoblastic 
  pole 
  of 
  the 
  blastula 
  with 
  yelk 
  causes 
  

   the 
  latter 
  to 
  be 
  constricted 
  around 
  its 
  equator 
  in 
  the 
  course 
  of 
  develop- 
  

   ment, 
  thus 
  leading 
  to 
  the 
  formation 
  of 
  a 
  blastodisk 
  with 
  an 
  inflected 
  

   two-layered 
  margin. 
  Secondly, 
  since 
  the 
  foregoing 
  is 
  true, 
  it 
  results 
  

   that 
  active 
  development 
  is 
  shifted 
  so 
  entirely 
  towards 
  one 
  pole 
  of 
  the 
  

   egg 
  that 
  gastrulation 
  also 
  occurs 
  there, 
  and 
  an 
  attempt 
  is 
  made.' 
  to 
  re- 
  

   produce 
  the 
  state 
  of 
  things 
  seen 
  in 
  the 
  gastrula 
  of 
  Branchiostoma,, 
  but 
  

   in 
  the 
  effort 
  of 
  the 
  active 
  pole 
  of 
  the 
  blastula 
  to 
  envelop 
  the 
  passive 
  

   one 
  by 
  epibole 
  it 
  appears 
  as 
  if 
  the 
  annular 
  entoblastic 
  invagination 
  was 
  

   incomplete, 
  leaving 
  the 
  " 
  Urmund" 
  of 
  Kollmann 
  open. 
  This 
  " 
  Urmund" 
  

   is 
  homologous 
  with 
  a 
  circular 
  opening 
  which 
  might 
  be 
  produced 
  by 
  a 
  

   rupture 
  near 
  the 
  center 
  of 
  the 
  inflected 
  entoblast 
  of 
  the 
  gastrula 
  of 
  

   Branchiostoma, 
  and 
  therefore 
  is 
  in 
  no 
  sense 
  homologous 
  with 
  the 
  true 
  

   blastopore. 
  The 
  opening 
  in 
  the 
  floor 
  of 
  the 
  discoblastula 
  of 
  Teleosts 
  

   and 
  Elasmobranchs, 
  identified 
  by 
  Kollmann 
  with 
  the 
  blastopore, 
  I 
  pro- 
  

   pose 
  to 
  call 
  the 
  discopore, 
  in 
  order 
  to 
  permanently 
  distinguish 
  it 
  from 
  

   the 
  true 
  blastopore 
  of 
  authors. 
  An 
  equally 
  fatal 
  objection 
  to 
  Koll- 
  

   mann's 
  view 
  is 
  that 
  on 
  the 
  basis 
  of 
  his 
  interpretations 
  the 
  blastopore 
  of 
  

   Teleosts 
  and 
  Selachians 
  would 
  not 
  open 
  to 
  the 
  exterior. 
  

  

  A 
  view 
  resembling 
  in 
  some 
  respects 
  that 
  of 
  Kollmann 
  was 
  enter- 
  

   tained 
  by 
  me 
  in 
  1881 
  (see 
  la, 
  supra, 
  p. 
  298), 
  though 
  it 
  was 
  immedi- 
  

   ately 
  qualified 
  in 
  paragraph 
  2 
  which 
  followed 
  on 
  the 
  same 
  page, 
  and 
  

   was 
  subsequently 
  adopted 
  in 
  a 
  modified 
  form 
  in 
  my 
  first 
  paper 
  on 
  Gadus 
  

   (Contr. 
  Embryog. 
  Oss. 
  Fishes, 
  p. 
  569). 
  I 
  had 
  also 
  observed 
  15 
  " 
  and 
  

   figured 
  the 
  segmentation 
  of 
  the 
  "nuclear 
  zone" 
  or 
  "marginal 
  wreath 
  of 
  

  

  15 
  Kolluiann. 
  Genieinsarne 
  Entwickelungsbahiien 
  der 
  Wirbelthiere. 
  Arch. 
  f. 
  Anat, 
  

   u. 
  Physiol., 
  1885. 
  Anat. 
  Abth, 
  pp. 
  279-306, 
  pi. 
  xii. 
  

   ; 
  5a 
  See 
  No. 
  7, 
  p. 
  146, 
  fig. 
  4, 
  pi, 
  1 
  ; 
  apd 
  also 
  No. 
  la, 
  p. 
  287, 
  fig. 
  3, 
  pi. 
  six, 
  

  

  