﻿28 
  G. 
  IIEHBKHT 
  FoWrJOlf. 
  

  

  iilimeutary 
  canal 
  began 
  to 
  diminish, 
  the 
  mesentery 
  to 
  elon- 
  

   gate 
  ; 
  and 
  in 
  fig. 
  14 
  the 
  alimentary 
  canal 
  appeared 
  to 
  be 
  

   represented 
  by 
  the 
  central 
  core 
  of 
  the 
  mesentery 
  of 
  tlie 
  

   gymnocaulus, 
  the 
  two 
  trnnk-cavities 
  becoming 
  the 
  paired 
  

   cavities 
  of 
  the 
  stalk. 
  

  

  I 
  have 
  endeavonred 
  to 
  express 
  my 
  interpretation 
  of 
  these 
  

   sections 
  by 
  an 
  imaginai'y 
  longitudinal 
  section 
  in 
  fig. 
  15. 
  If 
  my 
  

   views 
  are 
  correct, 
  two 
  things 
  follow 
  — 
  that 
  the 
  notochord 
  

   in 
  the 
  bud 
  is 
  of 
  ectodermal 
  origin, 
  and 
  that 
  the 
  

   gymnocaulus 
  contains 
  all 
  three 
  embryonic 
  laj^ers, 
  

   the 
  pi'oliferation 
  and 
  growth 
  of 
  which 
  give 
  rise 
  to 
  equivalent 
  

   structures 
  in 
  the 
  adult. 
  

  

  As 
  regards 
  the 
  notochord, 
  I 
  have 
  long 
  suspected 
  that 
  it 
  

   Avas 
  a 
  stomodasal 
  structure 
  in 
  Balanoglossus 
  and 
  Cephalo- 
  

   discus, 
  and 
  there 
  can 
  be 
  little 
  hesitation 
  in 
  assigning 
  it 
  to 
  

   the 
  ectoderm 
  in 
  buds 
  of 
  Rhabdopleura 
  on 
  the 
  st)ength 
  

   of 
  these 
  sections. 
  Figs. 
  7, 
  8, 
  and 
  9 
  show 
  an 
  epithelial 
  in- 
  

   vagination 
  below 
  the 
  proboscis-stalk, 
  which, 
  from 
  the 
  cha- 
  

   racter 
  of 
  the 
  cells, 
  is 
  fairly 
  certainly 
  ectodermal, 
  and 
  is 
  

   continuous 
  with 
  the 
  so-called 
  notochord; 
  the 
  idimentary 
  

   canal, 
  on 
  the 
  other 
  hand, 
  appears, 
  so 
  far 
  as 
  I 
  can 
  see, 
  to 
  be 
  

   syncytial 
  and 
  vacuolated 
  rather 
  than 
  epithelial 
  ; 
  this 
  is 
  shut 
  off 
  

   by 
  a 
  basement 
  membrane 
  from 
  the 
  stomodseum 
  at 
  the 
  jdane 
  

   of 
  these 
  sections, 
  and 
  is 
  presumably 
  the 
  future 
  endoderm. 
  

  

  As 
  regards 
  the 
  structure 
  of 
  the 
  adult 
  gymnocaulus, 
  I 
  liave 
  

   no 
  personal 
  doidjt 
  of 
  the 
  view 
  given 
  above, 
  that 
  the 
  contents 
  

   of 
  the 
  central 
  cavity 
  in 
  the 
  septum 
  are 
  continuous 
  with 
  the 
  

   alimentary 
  canal 
  of 
  the 
  adult, 
  and 
  give 
  rise 
  to 
  the 
  ali- 
  

   mentary 
  canal 
  of 
  the 
  bud; 
  they 
  are 
  presumably 
  of 
  endo- 
  

   dermal 
  origin. 
  Similarly 
  the 
  paired 
  cavities 
  of 
  the 
  gyui- 
  

   nocaulus 
  are 
  traceable 
  faii'ly 
  unmistaka,bly 
  into 
  the 
  trunk- 
  

   cavities 
  of 
  the 
  bud, 
  less 
  certainly 
  into 
  those 
  of 
  the 
  adult. 
  

   At 
  the 
  same 
  time, 
  the 
  structures 
  in 
  question 
  are 
  so 
  minute 
  

   that 
  these 
  views 
  have 
  only 
  the 
  value 
  of 
  a 
  personal 
  conviction, 
  

   and 
  require 
  confirmation 
  from 
  other 
  sources. 
  

  

  These 
  notes 
  and 
  drawings 
  of 
  the 
  structure 
  of 
  the 
  stalk 
  and 
  

   bud, 
  such 
  as 
  they 
  are, 
  were 
  made 
  before 
  the 
  publication 
  of 
  

  

  