﻿172 
  Mr. 
  A. 
  Sanders 
  on 
  the 
  

  

  On 
  comparing 
  a 
  section 
  through 
  the 
  olfactory 
  lobe 
  with 
  one 
  

   through 
  the 
  cerebrum, 
  it 
  is 
  possible 
  to 
  imagine 
  how 
  the 
  

   former 
  could 
  have 
  been 
  evolved 
  from 
  the 
  latter. 
  The 
  internal 
  

   layer 
  of 
  the 
  olfactory 
  lobe 
  resembles 
  in 
  every 
  respect 
  and 
  is 
  

   continuous 
  with 
  the 
  internal 
  layer 
  of 
  the 
  cerebrum, 
  the 
  second 
  

   and 
  third 
  layers 
  of 
  which 
  would 
  correspond 
  to 
  the 
  middle 
  

   layer 
  of 
  the 
  olfactory 
  lobe, 
  while 
  the 
  glomeruli 
  would 
  be 
  the 
  

   condensed 
  external 
  layer 
  of 
  the 
  cerebrum. 
  

  

  This 
  idea 
  is 
  supported 
  by 
  the 
  fact 
  that 
  the 
  three 
  internal 
  

   strata 
  of 
  the 
  cerebrum 
  equal 
  in 
  width 
  the 
  two 
  internal 
  layers 
  

   of 
  the 
  olfactory 
  lobe, 
  while 
  the 
  external 
  layer 
  of 
  the 
  former 
  is 
  

   much 
  wider 
  than 
  the 
  zone 
  of 
  glomeruli, 
  which 
  are 
  undoubt- 
  

   edly 
  formed 
  of 
  more 
  condensed 
  substance 
  than 
  the 
  neuroglia 
  

   of 
  the 
  cerebrum. 
  

  

  Hyjyoaria. 
  

  

  The 
  hypoaria 
  (PI. 
  IX. 
  fig. 
  6) 
  as 
  distinct 
  structures 
  seem 
  to 
  

   be 
  represented 
  in 
  Ceratodus 
  only 
  by 
  the 
  laminae 
  of 
  nervous 
  

   tissue 
  which 
  form 
  the 
  walls 
  of 
  the 
  infundibulum. 
  

  

  Their 
  structure 
  resembles 
  that 
  of 
  the 
  cerebrum, 
  that 
  is 
  to 
  

   say, 
  there 
  exists 
  a 
  layer 
  of 
  cells 
  on 
  the 
  inner 
  surface, 
  and 
  

   external 
  to 
  this 
  the 
  parenchyma 
  is 
  occupied 
  by 
  neuroglia, 
  in 
  

   which 
  a 
  fine 
  network 
  of 
  fibrillse 
  is 
  imbedded. 
  

  

  The 
  larger 
  cells 
  which 
  characterize 
  the 
  third 
  layer 
  of 
  the 
  

   cerebral 
  walls 
  are 
  here 
  absent. 
  

  

  Lobi 
  optici. 
  

  

  The 
  optic 
  lobes 
  (PL 
  XII. 
  fig. 
  8, 
  PI. 
  XIII. 
  fig. 
  23) 
  have 
  a 
  

   structure 
  comparable 
  to 
  that 
  of 
  the 
  optic 
  lobes 
  of 
  the 
  Plagio- 
  

   stomata. 
  On 
  the 
  internal 
  surface 
  there 
  is 
  seen 
  the 
  layer 
  of 
  

   endothelial 
  cells 
  which 
  line 
  the 
  aqueduct 
  of 
  Sylvius 
  and 
  are 
  

   continuous 
  with 
  the 
  endothelium 
  of 
  the 
  remaining 
  cavities 
  of 
  

   the 
  brain. 
  

  

  External 
  to 
  the 
  endothelium 
  four 
  layers 
  may 
  be 
  distin- 
  

   guished 
  in 
  this 
  structure. 
  

  

  The 
  first 
  is 
  a 
  layer 
  of 
  cells 
  the 
  great 
  majority 
  of 
  which 
  are 
  

   spherical, 
  but 
  many 
  are 
  pyriform 
  or 
  oval 
  ; 
  each 
  sends 
  a 
  pro- 
  

   cess 
  from 
  one 
  side 
  in 
  a 
  radial 
  direction, 
  which 
  may 
  be 
  traced 
  

   into 
  the 
  parenchyma 
  of 
  the 
  interior 
  of 
  the 
  lobe 
  ; 
  the 
  cells 
  in 
  

   this 
  layer 
  show 
  a 
  tendency 
  to 
  be 
  arranged 
  in 
  clusters 
  on 
  these 
  

   radial 
  processes, 
  in 
  the 
  same 
  way 
  as 
  in 
  the 
  corresponding 
  

   layer 
  of 
  the 
  tectum 
  lobi 
  optici 
  in 
  Teleostei, 
  but 
  the 
  arrange- 
  

   ment 
  is 
  not 
  so 
  well 
  marked 
  here 
  ; 
  many 
  of 
  them 
  resemble 
  the 
  

  

  