﻿DEVELOPMENT 
  OF 
  THE 
  DIGESTIVE 
  CANAL. 
  267 
  

  

  terior, 
  called 
  the 
  proctodceum, 
  becomes 
  the 
  auus 
  audrectam. 
  The 
  mid- 
  

   dle 
  portion 
  of 
  the 
  tract 
  includes 
  the 
  stomach, 
  ending 
  just 
  in 
  front 
  of 
  

   the 
  urinary 
  tubes, 
  which 
  open 
  into 
  the 
  anterior 
  end 
  of 
  the 
  proctodwum. 
  

  

  PI. 
  XVI, 
  Fig. 
  1, 
  shows 
  the 
  embryo 
  of 
  the 
  locust 
  with 
  the 
  stomodseum 
  

   and 
  proctodseum 
  well 
  developed, 
  the 
  middle 
  portion 
  not 
  being 
  yet 
  de- 
  

   veloped 
  from 
  the 
  yolk. 
  The 
  fore 
  and 
  hind 
  intestine 
  originally 
  arise 
  by 
  

   inpushings 
  or 
  invaginations 
  of 
  the 
  ectoblast, 
  and 
  they 
  are 
  distinguished 
  

   by 
  being 
  lined 
  with 
  chitine. 
  

  

  The 
  stomach 
  or 
  mesenteron 
  is 
  at 
  first 
  a 
  closed 
  sac 
  independent 
  of 
  

   the 
  fore 
  and 
  hind 
  divisions 
  of 
  the 
  alimentary 
  tract. 
  It 
  finally 
  touches 
  

   the 
  opposing 
  ends 
  of 
  the 
  stomodseum 
  and 
  i3roctod3Bum 
  5 
  the 
  cellular 
  walls 
  

   are 
  absorbed, 
  disappear, 
  and 
  just 
  before 
  hatching 
  the 
  digestive 
  canal 
  

   becomes 
  a 
  continuous 
  hollow 
  tube 
  open 
  from 
  mouth 
  to 
  vent. 
  The 
  exact 
  

   origin 
  of 
  the 
  mesenteron 
  is 
  not 
  known 
  beyond 
  the 
  fact 
  that 
  the 
  stomach 
  

   is 
  lined 
  with 
  the 
  endoblast. 
  According 
  to 
  Kowalevsky, 
  in 
  Hydrophilus 
  

   the 
  mesoblast 
  is 
  concerned 
  in 
  the 
  formation 
  of 
  the 
  stomach. 
  At 
  any 
  rate, 
  

   the 
  yolk- 
  cells 
  appear 
  to 
  give 
  rise 
  to 
  the 
  endoblastic 
  walls 
  of 
  the 
  stomach 
  

   (mesenteron). 
  

  

  The 
  coecal 
  or 
  pyloric 
  appendages 
  of 
  the 
  stomach 
  have 
  the 
  same 
  ori- 
  

   gin 
  as 
  the 
  latter; 
  while 
  the 
  Malpighian 
  or 
  -urinary 
  tubes 
  arise, 
  as 
  Dohrn 
  

   first 
  observed 
  in 
  the 
  embryo 
  mole 
  cricket, 
  and 
  as 
  we 
  shall 
  see 
  in 
  the 
  

   locust 
  (PI. 
  XXI, 
  Fig. 
  6, 
  ut), 
  as 
  outgrowths 
  of 
  the 
  ectoblast 
  of 
  the 
  anterior 
  

   end 
  of 
  the 
  proctodseum. 
  

  

  The 
  origin 
  of 
  the 
  genital 
  glands 
  has 
  not 
  been 
  clearly 
  made 
  out. 
  The 
  

   polar 
  cells 
  which 
  appear 
  in 
  Diptera 
  before 
  any 
  other 
  cells 
  of 
  the 
  blasto- 
  

   derm, 
  those 
  referred 
  to 
  in 
  the 
  note 
  on 
  p. 
  263, 
  are 
  supposed 
  to 
  give 
  origin 
  

   to 
  the 
  ovaries 
  or 
  testes. 
  Dohrn 
  states 
  that 
  in 
  the 
  mole 
  cricket 
  the 
  geni- 
  

   tal 
  glands 
  arise 
  from 
  the 
  proctodseum. 
  As 
  Balfour 
  states 
  in 
  his 
  ^' 
  Com- 
  

   parative 
  Embryology," 
  it 
  appears 
  to 
  be 
  fairly 
  clear 
  that 
  the 
  genital 
  

   glands 
  of 
  both 
  sexes 
  have 
  an 
  identical 
  origin. 
  

  

  The 
  trachew 
  and 
  salivary 
  glands. 
  — 
  As 
  may 
  be 
  seen 
  in 
  PI. 
  XVI, 
  Fig. 
  1, 
  

   these 
  arise 
  nearly 
  simultaneously. 
  The 
  tracheae 
  and 
  stigmata 
  arise 
  as 
  

   Invaginations 
  of 
  the 
  ectoblast; 
  the 
  stigmata 
  are 
  the 
  original 
  or 
  primi- 
  

   tive 
  cavities 
  caused 
  by 
  the 
  ingrowth 
  of 
  the 
  walls 
  of 
  the 
  primitive 
  band, 
  

   and 
  there 
  is 
  a 
  pair 
  to 
  each 
  segment, 
  the 
  head- 
  segments 
  (except 
  the 
  an- 
  

   tennal) 
  perhaps 
  not 
  excepted. 
  The 
  three 
  thoracic 
  and 
  first 
  eight 
  ab- 
  

   dominal 
  segments 
  are 
  provided 
  each 
  with 
  a 
  pair 
  of 
  stigmata. 
  The 
  

   blind 
  ends 
  of 
  the 
  tubes 
  unite 
  in 
  the 
  locust 
  to 
  form 
  on 
  each 
  side 
  a 
  dor- 
  

   sal 
  and 
  ventral 
  longitudinal 
  main 
  trachea. 
  

  

  The 
  salivary 
  glands 
  (PL 
  XVI, 
  Fig. 
  1, 
  sal)^ 
  are 
  said 
  to 
  be 
  ectoblastic 
  

   invaginations, 
  and 
  to 
  be 
  formed 
  in 
  the 
  same 
  general 
  manner 
  as 
  the 
  

   tracheae 
  ; 
  they 
  are 
  not 
  invaginations 
  of 
  the 
  stomodaeum, 
  but, 
  as 
  Balfour 
  

   states, 
  arise 
  from 
  the 
  primitive 
  band 
  behind 
  the 
  mouth 
  on 
  the 
  inner 
  side 
  

   of 
  the 
  mandibles. 
  

  

  The 
  spinning 
  glands 
  of 
  caterpillars 
  and 
  of 
  bee 
  larvae 
  (Apis) 
  arise 
  in 
  

   the 
  same 
  way, 
  but 
  are 
  situated 
  on 
  the 
  inner 
  side 
  of 
  the 
  first 
  maxillae. 
  

  

  