﻿238 
  REPORT 
  UNITED 
  STATES 
  ENTOMOLOGICAL 
  COMMISSION. 
  

  

  As 
  the 
  fibers 
  of 
  the 
  adult 
  brain 
  are 
  evidently 
  secondary 
  products, 
  it 
  

   would 
  appear 
  that 
  they 
  must 
  be 
  transformed 
  granules 
  or 
  nuclei, 
  and 
  

   not 
  in 
  all 
  cases, 
  at 
  least, 
  the 
  fibers 
  thrown 
  off 
  from 
  the 
  ganglion 
  cells, 
  

   although 
  at 
  this 
  time 
  the 
  ganglion 
  cells 
  have 
  no 
  fibers, 
  the 
  fibers 
  of 
  

   those 
  seen 
  in 
  the 
  adult 
  brain 
  being 
  also 
  secondary 
  growths. 
  It 
  may 
  be 
  

   that 
  the 
  white 
  inner 
  granulo-fibrous 
  matter 
  of. 
  the 
  adult 
  brain 
  is 
  (1) 
  

   made 
  up 
  of 
  modified 
  granules, 
  which 
  in 
  some 
  cases 
  remain 
  such, 
  and 
  in 
  

   others 
  form 
  fibers, 
  and 
  (2) 
  of 
  fibers 
  sent 
  in 
  from 
  the 
  cortical 
  ganglion 
  

   cells. 
  

  

  Comparison 
  of 
  the 
  brain 
  at 
  this 
  stage 
  with 
  the 
  first 
  thoracic 
  ganglion. 
  — 
  

   If 
  we 
  compare 
  at 
  this 
  stage 
  of 
  development 
  of 
  the 
  nervous 
  system 
  the 
  

   brain 
  with 
  one 
  of 
  the 
  ganglia 
  of 
  the 
  trunk, 
  we 
  shall 
  obtain 
  a 
  fair 
  idea 
  

   of 
  the 
  primitive 
  difference 
  between 
  the 
  brain 
  and 
  one 
  of 
  the 
  ordinary 
  

   ganglia 
  (PL 
  XIII, 
  Fig. 
  5). 
  By 
  a 
  glance 
  at 
  the 
  figures 
  of 
  the 
  two 
  it 
  will 
  

   be 
  seen 
  that 
  the 
  organization 
  of 
  the 
  thoracic 
  ganglion 
  is 
  essentially 
  

   simple. 
  It 
  is 
  divided 
  into 
  two 
  portions 
  or 
  regions. 
  The 
  central 
  granular 
  

   region 
  is 
  enveloped 
  by 
  a 
  thick 
  stratum 
  of 
  cortical 
  ganglion 
  cells. 
  The 
  

   whole 
  ganglion 
  in 
  section 
  is 
  rudely 
  hour-glass-shaped 
  and 
  much 
  smaller 
  

   than 
  the 
  brain. 
  There 
  is 
  no 
  differentiation 
  into 
  distinct 
  lobes 
  as 
  in 
  the 
  

   brain. 
  The 
  formation 
  of 
  the 
  brain, 
  as 
  is 
  well 
  known 
  by 
  embryologists, 
  

   is 
  one 
  of 
  the 
  earliest 
  steps 
  in 
  the 
  development 
  of 
  the 
  nervous 
  system, 
  

   the 
  entire 
  system 
  being 
  at 
  an 
  early 
  date 
  in 
  the 
  life 
  of 
  the 
  embryo 
  set 
  

   apart 
  from 
  the 
  epidermis 
  or 
  integument, 
  the 
  latter 
  with 
  the 
  nervous 
  

   system 
  originating 
  from 
  the 
  ectoderm 
  or 
  outer 
  germ-layer. 
  

  

  Second 
  embryonic 
  stage, 
  B 
  (PI. 
  XII, 
  Figs. 
  2-9). 
  — 
  In 
  embryos 
  more 
  ad- 
  

   vanced, 
  and 
  just 
  ready 
  to 
  hatch, 
  the 
  eyes 
  being 
  now 
  dark 
  red, 
  the 
  cen- 
  

   tral 
  body 
  is 
  formed, 
  but 
  our 
  sections 
  do 
  not 
  show 
  any 
  traces 
  of 
  a 
  mush- 
  

   room 
  body. 
  The 
  sections 
  are 
  frontal, 
  and 
  we 
  will 
  describe 
  them 
  in 
  

   order. 
  The 
  fifth 
  section 
  is 
  through 
  the 
  head 
  and 
  front 
  part 
  of 
  the 
  eyes, 
  

   but 
  does 
  not 
  graze 
  the 
  brain 
  itself. 
  Fig. 
  2 
  shows 
  the 
  structure 
  of 
  the 
  

   interior 
  of 
  the 
  head, 
  being 
  filled 
  with 
  connective 
  tissue 
  cells 
  not 
  dis- 
  

   tinguishable 
  from 
  the 
  ganglion 
  cells. 
  

  

  Section 
  6 
  (Fig. 
  3) 
  passes 
  through 
  the 
  outer 
  portion 
  of 
  the 
  optic 
  and 
  

   antennal 
  lobes, 
  now 
  clearly 
  differentiated. 
  

  

  In 
  section 
  7 
  the 
  cerebral 
  Jobes 
  are 
  seen, 
  and 
  in 
  section 
  8 
  are 
  larger, 
  as 
  

   are 
  the 
  optic 
  lobes, 
  while 
  the 
  antennal 
  lobes 
  are 
  somewhat 
  reduced 
  in 
  

   size. 
  Section 
  10 
  passes 
  through 
  the 
  cerebral 
  lobes 
  and 
  also 
  grazes 
  the 
  

   optic 
  lobes, 
  passing 
  through 
  the 
  optic 
  ganglion. 
  

  

  Section 
  11 
  (Fig. 
  8) 
  shows 
  the 
  central 
  body, 
  separated 
  from 
  the 
  upper 
  

   cerebral 
  lobes 
  by 
  a 
  thin 
  layer 
  of 
  loose 
  ganglionic 
  cells. 
  The 
  relation 
  of 
  

   the 
  central 
  body 
  to 
  the 
  upper 
  and 
  lower 
  cerebral 
  lobes 
  is 
  well 
  shown 
  in 
  

   this 
  section. 
  

  

  Plate 
  XIII, 
  Fig. 
  4, 
  shows 
  the 
  relation 
  of 
  these 
  and 
  their 
  structure 
  

   greatly 
  enlarged. 
  Through 
  the 
  granular 
  substance 
  of 
  the 
  lobes 
  are 
  

   sparsely 
  scattered 
  ganglionic 
  cells. 
  

  

  Section 
  12 
  passes 
  through 
  the 
  lower 
  cerebral 
  lobes 
  and 
  the 
  upper 
  left 
  

  

  