﻿[55] 
  DEVELOPMENT 
  OF 
  OSSEOUS 
  FISHES. 
  543 
  

  

  possible 
  that 
  the 
  so-called 
  atlnasal 
  of 
  McMurrich, 
  54 
  may, 
  if 
  not 
  actually 
  

   a 
  part 
  of 
  the 
  suborbital 
  chain, 
  as 
  he 
  surmises, 
  in 
  reality 
  represent 
  the 
  

   maxillary 
  of 
  other 
  fishes, 
  since 
  this 
  adnasal 
  element 
  is 
  clearly 
  a 
  mem- 
  

   brane 
  bone, 
  while 
  it 
  is 
  not 
  altogether 
  certain 
  that 
  the 
  so-called 
  "max- 
  

   illary" 
  of 
  the 
  Nematognathi 
  can 
  be 
  considered 
  such. 
  While 
  the 
  ossifica- 
  

   tion 
  of 
  the 
  upper 
  end 
  of 
  the 
  cartilaginous 
  bar 
  Mxb 
  is 
  superficial, 
  old 
  

   specimens 
  of 
  Ictalurus 
  show 
  that 
  the 
  cartilage 
  is 
  invaded 
  and 
  replaced 
  

   by 
  the 
  process, 
  so 
  that 
  its 
  terminal 
  portion 
  only 
  remains 
  cartilaginous. 
  

   The 
  other 
  barbels, 
  viz, 
  the 
  nasal 
  and 
  the 
  mental, 
  are 
  also 
  occasionally 
  

   ossified 
  at 
  the 
  base, 
  especially 
  in 
  old 
  specimens. 
  Both 
  the 
  internal 
  

   pair 
  of 
  mental 
  barbels 
  lb 
  and 
  the 
  external 
  pair 
  Eb 
  are 
  at 
  first 
  laid 
  down 
  

   in 
  cartilage 
  in 
  the 
  embryo. 
  The 
  strongest 
  argument 
  in 
  favor 
  of 
  regard- 
  

   ing 
  the 
  ossified 
  basal 
  parts 
  of 
  the 
  lateral 
  barbels 
  as 
  maxillary 
  elements 
  

   is 
  derived 
  from 
  a 
  study 
  of 
  the 
  distribution 
  of 
  the 
  branches 
  of 
  the 
  fifth 
  

   group 
  of 
  nerves 
  as 
  worked 
  out 
  by 
  Wright, 
  55 
  though 
  it 
  must 
  not 
  be 
  for- 
  

   gotten 
  that 
  these 
  organs 
  in 
  Siluroids 
  are 
  specialized 
  as 
  tactile 
  organs, 
  

   and 
  that 
  they 
  may 
  therefore 
  be 
  richly 
  supplied 
  with 
  nerves, 
  in 
  corre- 
  

   spondence 
  with 
  their 
  high 
  degree 
  of 
  specialization. 
  

  

  A 
  longitudinal, 
  median, 
  vertical 
  section 
  through 
  the 
  head 
  of 
  Ictalurus 
  

   is 
  represented 
  in 
  fig. 
  174, 
  prepared 
  from 
  an 
  embryo 
  of 
  the 
  same 
  age 
  

   as 
  that 
  used 
  in 
  working 
  out 
  the 
  cranium 
  represented 
  in 
  the 
  preceding 
  

   figure. 
  The 
  brain 
  is 
  shown 
  in 
  mesial 
  section 
  and 
  illustrates 
  the 
  rela- 
  

   tions 
  of 
  the 
  cerebrum 
  Cer 
  anteriorly 
  to 
  the 
  pineal 
  body 
  Pn 
  just 
  behind 
  

   it. 
  The 
  narrow 
  midbrain 
  mb 
  is 
  also 
  shown, 
  and 
  upon 
  which 
  the 
  re- 
  

   markably 
  voluminous 
  cerebellum 
  Cb 
  encroaches 
  from 
  behind. 
  At 
  an 
  

   earlier 
  stage 
  the 
  great 
  antero-posterior 
  width 
  of 
  the 
  cerebellum 
  is 
  far- 
  

   less 
  obvious, 
  so 
  that 
  it 
  is 
  quite 
  clear 
  that 
  the 
  excessive 
  anterior 
  exten- 
  

   sion 
  of 
  the 
  cerebellum 
  in 
  the 
  Siluroids 
  is 
  a 
  result 
  of 
  the 
  exaggerated 
  

   development 
  or 
  specialization 
  of 
  this 
  portion 
  of 
  the 
  brain 
  of 
  the 
  ordi- 
  

   nary 
  Teleostean 
  type. 
  The 
  medulla 
  oblongata 
  mo 
  is 
  massive. 
  The 
  in- 
  

   fundibulum 
  Inf 
  departs 
  but 
  little 
  in 
  its 
  form 
  from 
  that 
  usually 
  met 
  with 
  

   in 
  the 
  embryos 
  of 
  osseous 
  fishes. 
  The 
  cranial 
  nerves 
  and 
  brain 
  of 
  

   Amiurus 
  has 
  been 
  so 
  carefully 
  described 
  by 
  Wright 
  (op. 
  cit.) 
  that 
  no 
  

   further 
  discussion 
  of 
  this 
  part 
  of 
  the 
  subject 
  will 
  be 
  entered 
  upon 
  here, 
  

   except 
  to 
  call 
  attention 
  to 
  the 
  disposition 
  of 
  the 
  sacculus 
  vasculosus 
  8 
  

   and 
  the 
  hypophysis 
  My. 
  

  

  A 
  mesial 
  section 
  of 
  the 
  heart 
  is 
  also 
  displayed 
  in 
  fig. 
  174. 
  The 
  thin- 
  

   walled 
  sinus 
  venosus 
  S 
  V, 
  the 
  muscular 
  ventricle 
  Fc, 
  and 
  the 
  bulbus 
  aortoe 
  

   Ba, 
  have 
  been 
  cut 
  through. 
  The 
  tip 
  of 
  the 
  liver 
  i, 
  crowded 
  into 
  a 
  cav- 
  

   ity 
  in 
  the 
  coarsely 
  granular 
  yelk 
  F, 
  is 
  also 
  shown, 
  together 
  with 
  the 
  more 
  

   homogeneous 
  periblast 
  P, 
  which 
  invests 
  the 
  mass 
  of 
  granular 
  deuto- 
  

   plasm. 
  The 
  granules 
  of 
  deutoplasm 
  in 
  the 
  yelk-sack 
  are 
  characteris- 
  

  

  51 
  J. 
  P. 
  McMurrich. 
  Osteology 
  of 
  Amiurus, 
  Proc. 
  Canadian 
  Inst. 
  Toronto. 
  N. 
  S., 
  ii, 
  

   No. 
  3, 
  p. 
  278, 
  pi. 
  ii, 
  fig. 
  1, 
  An. 
  

  

  65 
  R. 
  R. 
  Wright. 
  On 
  the 
  nervous 
  system 
  and 
  sense 
  organs 
  of 
  Amiurus. 
  Proc. 
  Cana- 
  

   dian 
  Inst. 
  Toronto. 
  N. 
  S. 
  ; 
  ii, 
  No. 
  3, 
  pp. 
  3GG-3G8, 
  pi. 
  iv. 
  

  

  