﻿266 
  REPORT 
  UNITED 
  STATES 
  ENTOMOLOGICAL 
  COMMISSION. 
  

  

  Division 
  of 
  the 
  embryo 
  or 
  primitive 
  hand 
  into 
  body 
  -segments. 
  — 
  Mean- 
  

   while 
  the 
  primitive 
  band 
  grows 
  at 
  the 
  expense 
  of 
  the 
  yolk, 
  spreading 
  

   out 
  more 
  and 
  more 
  over 
  its 
  surface, 
  until 
  (see 
  PL 
  XXII, 
  Figs. 
  1, 
  2) 
  it 
  lies 
  

   like 
  a 
  broad 
  riband 
  over 
  the 
  yolk, 
  each 
  end 
  nearly 
  meeting. 
  By 
  this 
  

   time 
  it 
  becomes 
  divided 
  by 
  transverse 
  impressed 
  lines 
  into 
  segments, 
  

   which 
  correspond 
  to 
  those 
  of 
  the 
  larva 
  and 
  adult. 
  The 
  first 
  of 
  these 
  seg- 
  

   ments 
  is 
  divided 
  into 
  two 
  broad 
  and 
  flaring 
  flai)s, 
  which 
  are 
  called 
  the 
  

   procephalic 
  lobes. 
  It 
  becomes 
  the 
  antennal 
  segment 
  j 
  behind 
  this 
  are 
  

   16 
  segments, 
  making 
  17 
  in 
  all, 
  or, 
  as 
  in 
  many 
  insects 
  (locust, 
  &c.), 
  which 
  

   have 
  11 
  abdominal 
  segments, 
  there 
  are 
  18 
  body-segments, 
  4 
  for 
  the 
  head, 
  

   3 
  for 
  the 
  thorax, 
  and 
  11 
  for 
  the 
  abdomen. 
  

  

  Development 
  of 
  the 
  appendages. 
  — 
  ISTearly 
  it' 
  not 
  quite 
  simultaneously 
  all 
  

   the 
  limbs 
  bud 
  out 
  from 
  each 
  side 
  of 
  the 
  median 
  line 
  of 
  the 
  primitive 
  

   band. 
  There 
  are 
  never 
  more 
  than 
  one 
  pair 
  to 
  each 
  segment 
  j 
  in 
  some 
  

   insects 
  there 
  are, 
  as 
  in 
  the 
  Lepidoptera, 
  a 
  pair 
  on 
  each 
  abdominal 
  seg- 
  

   ment. 
  The 
  three 
  pairs 
  of 
  rhabdites 
  or 
  elements 
  of 
  the 
  ovipositor 
  in 
  

   bees, 
  grasshoppers, 
  &c., 
  arise 
  in 
  the 
  same 
  manner 
  as 
  limbs. 
  It 
  will 
  

   be 
  noticed, 
  as 
  seen 
  in 
  PI. 
  XVII, 
  Figs. 
  1, 
  la, 
  that 
  the 
  antennae, 
  jaws 
  and 
  

   maxillae, 
  as 
  well 
  as 
  legs, 
  are 
  alike 
  at 
  first. 
  They 
  acquire 
  their 
  definitive 
  

   form 
  just 
  before 
  the 
  embryo 
  hatches 
  from 
  the 
  egg. 
  

  

  In 
  some 
  insects, 
  as 
  the 
  flea, 
  the 
  embryo 
  is 
  provided 
  with 
  a 
  temporary 
  

   knife-shaped 
  process 
  on 
  top 
  of 
  the 
  head, 
  called 
  the 
  egg-shell 
  cutter, 
  for 
  

   the 
  purpose 
  of 
  splitting 
  open 
  the 
  tough 
  egg-shell 
  that 
  the 
  embryo 
  may 
  

   escape. 
  

  

  Turning 
  now 
  to 
  the 
  internal 
  changes 
  during 
  the 
  development 
  of 
  the 
  

   embryo, 
  we 
  have 
  to 
  trace 
  the 
  formation 
  of 
  the 
  digestive 
  cavity, 
  the 
  

   nervous 
  system, 
  heart, 
  &c. 
  Of 
  the 
  different 
  viscera, 
  the 
  first 
  to 
  appear 
  

   after 
  the 
  formation 
  of 
  the 
  blastodermic 
  disk 
  or 
  primitive 
  band 
  is 
  the 
  

  

  Nervous 
  system. 
  — 
  This 
  develops 
  in 
  the 
  ectoblast 
  or 
  outer 
  germinal 
  cell- 
  

   layer. 
  It 
  arises 
  as 
  two 
  cellular 
  strings 
  or 
  cords 
  which 
  split 
  off 
  from 
  the 
  skin. 
  

   They 
  lie 
  one 
  on 
  each 
  side 
  of 
  the 
  median 
  line 
  of 
  the 
  bod^^ 
  They 
  are 
  enlarged 
  

   at 
  intervals 
  corresponding 
  to 
  the 
  segments 
  of 
  the 
  body, 
  forming 
  a 
  series 
  

   of 
  large 
  ganglia, 
  which 
  lie 
  next 
  to 
  each 
  other, 
  and 
  are 
  connected 
  by 
  

   short 
  strands 
  or 
  commissures. 
  As 
  seen 
  in 
  PL 
  XYII, 
  Fig. 
  2, 
  there 
  are 
  in 
  

   the 
  locust 
  15 
  ganglia, 
  or 
  one 
  to 
  each 
  segment 
  of 
  the 
  body 
  except 
  the 
  

   mandibular 
  and 
  maxillary 
  segments 
  in 
  the 
  head. 
  The 
  nervous 
  system 
  

   may 
  be 
  divided 
  into 
  two 
  parts, 
  one 
  the 
  brain, 
  which 
  is 
  prseoral, 
  situated 
  

   in 
  the 
  upper 
  part 
  of 
  the 
  head, 
  while 
  the 
  rest 
  of 
  the 
  system 
  is 
  postoral 
  

   and 
  lies 
  on 
  the 
  ventral 
  side 
  of 
  the 
  body. 
  In 
  section 
  each 
  ganglion 
  con- 
  

   sists 
  at 
  first 
  of 
  nucleated 
  cells, 
  those 
  in 
  the 
  center 
  being 
  more 
  close 
  and 
  

   dense 
  than 
  those 
  on 
  the 
  outside. 
  

  

  The 
  alimentary 
  canal. 
  — 
  This 
  does 
  not 
  arise 
  at 
  once, 
  the 
  middle 
  portion 
  

   not 
  being 
  developed 
  until 
  just 
  before 
  the 
  embryo 
  hatches. 
  The 
  aliment- 
  

   ary 
  canal 
  in 
  embryo 
  insects 
  is 
  divided 
  into 
  three 
  primary 
  portions. 
  The 
  

   anterior, 
  called 
  the 
  stomodceum, 
  becomes 
  differentiated 
  into 
  the 
  pharynx, 
  

   cesophagus, 
  crop 
  (ingluvies), 
  and 
  proventriculus 
  of 
  the 
  adult 
  ) 
  the 
  pos- 
  

  

  