﻿[41] 
  DEVELOPMENT 
  OF 
  OSSEOUS 
  FISHES. 
  529 
  

  

  nection 
  with 
  the 
  last-named 
  layer 
  longest 
  at 
  its 
  posterior 
  extremity, 
  as 
  

   shown 
  in 
  fig. 
  98. 
  After 
  the 
  tail 
  begins 
  to 
  bud 
  out, 
  however, 
  its 
  inti- 
  

   mate 
  connection 
  with 
  the 
  hypoblastic 
  layer 
  is 
  broken, 
  and 
  it 
  then 
  termi- 
  

   nates 
  after 
  becoming 
  somewhat 
  enlarged 
  in 
  the 
  cellular 
  terminal 
  mass 
  

   in 
  which 
  the 
  lateral 
  myoblastic 
  m, 
  neural 
  JV 
  r 
  , 
  and 
  post 
  anal 
  section 
  of 
  the 
  

   gut 
  % 
  terminate, 
  as 
  shown 
  in 
  cross-section 
  in 
  fig. 
  104, 
  and 
  in 
  vertical 
  and 
  

   transverse 
  optic 
  section 
  in 
  figs. 
  103 
  and 
  113. 
  The 
  histological 
  differentia- 
  

   tion 
  of 
  the 
  chorda 
  has 
  already 
  been 
  described 
  by 
  Knpffer, 
  Kowalevsky, 
  

   and 
  others, 
  so 
  that 
  there 
  is 
  no 
  occasion 
  for 
  me 
  to 
  redescribe 
  it, 
  as 
  it 
  takes 
  

   place 
  in 
  very 
  nearly 
  the 
  same 
  way 
  in 
  the 
  embryos 
  of 
  all 
  Chordata. 
  The 
  

   cells 
  of 
  the 
  chorda 
  of 
  Clupea 
  are, 
  however, 
  unusually 
  large 
  and 
  contain 
  

   very 
  spacious 
  cavities, 
  in 
  which 
  no 
  coagulable 
  albumen 
  is 
  present. 
  

  

  The 
  later 
  stages 
  of 
  the 
  development 
  of 
  Clupea 
  involve 
  mainly 
  the 
  

   completion 
  of 
  structures, 
  the 
  rudiments 
  of 
  which 
  were 
  laid 
  down 
  during 
  

   the 
  evolution 
  of 
  the 
  stages 
  already 
  described. 
  

  

  The 
  renal 
  apparatus 
  of 
  the 
  larval 
  shad 
  is 
  extremely 
  simple, 
  and 
  con- 
  

   sists, 
  at 
  the 
  time 
  the 
  tail 
  begins 
  to 
  bud 
  out, 
  of 
  a 
  pair 
  of 
  parallel 
  tubes 
  

   differentiated 
  from 
  before 
  backwards 
  from 
  the 
  outer 
  portions 
  of 
  the 
  

   mesoblast 
  at 
  the 
  time 
  it 
  splits 
  into 
  somatopleure 
  and 
  splanchnopleure. 
  

   The 
  segmental 
  ducts 
  sd 
  finally 
  lie 
  just 
  above 
  the 
  somatopleural 
  peri- 
  

   toneum 
  and 
  extend 
  from 
  a 
  little 
  way 
  behind 
  the 
  pectoral 
  plate 
  pp, 
  fig. 
  

   113, 
  or 
  the 
  pectoral 
  fins, 
  fig. 
  148, 
  to 
  a 
  point 
  just 
  behind 
  the 
  vent 
  where 
  

   they 
  debouch 
  into 
  a 
  common 
  cavity, 
  the 
  urinary 
  bladder 
  which 
  opens 
  

   outward 
  behind 
  the 
  vent. 
  At 
  their 
  anterior 
  extremities 
  the 
  segmental 
  

   ducts 
  terminate 
  in 
  a 
  single 
  nephridial 
  funnel 
  which 
  opens 
  into 
  the 
  body 
  

   cavity, 
  the 
  mouths 
  of 
  the 
  pair 
  of 
  funnels 
  being 
  directed 
  backward 
  and 
  

   inward, 
  so 
  that 
  the 
  anterior 
  extremities 
  of 
  the 
  ducts 
  are 
  bent 
  upon 
  

   themselves 
  in 
  the 
  form 
  of 
  a 
  shepherd's 
  crook. 
  

  

  The 
  development 
  of 
  the 
  cranium 
  is 
  not 
  precocious, 
  and 
  its 
  primary 
  

   cartilaginous 
  elements 
  are 
  not 
  very 
  apparent 
  until 
  after 
  hatching. 
  The 
  

   most 
  obvious 
  portions 
  are 
  the 
  trabecule 
  Tr, 
  figs. 
  142, 
  143, 
  and 
  144, 
  

   the 
  branchial 
  bars 
  i, 
  ii, 
  Hi, 
  iv, 
  and 
  v, 
  and 
  the 
  hyomandibular 
  arch, 
  com- 
  

   posed 
  of 
  the 
  hyomandibular 
  Hm, 
  interhyal 
  III, 
  quadrate 
  Q, 
  symplectic 
  

   Sy, 
  Meckel's 
  cartilage 
  Ml; 
  and 
  the 
  cerato- 
  and 
  glosso-hyal 
  elements 
  Ch 
  

   and 
  Gli. 
  The 
  auditory 
  vesicles 
  An 
  are 
  quite 
  large, 
  but 
  are 
  not 
  entirely 
  

   invested 
  by 
  cartilage 
  ; 
  only 
  the 
  outer 
  and 
  inferior 
  aspects 
  being 
  closed 
  in 
  

   by 
  chondrified 
  tissue, 
  as 
  shown 
  in 
  cross-section 
  in 
  fig. 
  147, 
  which 
  was 
  

   prepared 
  from 
  a 
  larva 
  about 
  six 
  days 
  old. 
  Fig. 
  144 
  represents 
  a 
  very 
  

   nearly 
  mesial 
  longitudinal 
  section 
  through 
  the 
  head 
  of 
  a 
  just-hatched 
  

   larva, 
  in 
  which 
  the 
  positions 
  of 
  the 
  cranial 
  cartilages 
  crossing 
  the 
  

   middle 
  line 
  are 
  indicated. 
  Fig. 
  143 
  represents 
  the 
  cranial 
  cartilages 
  

   of 
  a 
  slightly 
  older 
  larva 
  constructed 
  from 
  a 
  series 
  of 
  sections. 
  Fig. 
  

   142 
  represents 
  the 
  cranial 
  cartilages 
  of 
  a 
  still 
  more 
  advanced 
  larva, 
  in 
  

   which 
  the 
  antorbital 
  process 
  Ao 
  and 
  trabecular 
  rostrum 
  R 
  are 
  more 
  

   strongly 
  developed. 
  The 
  branchial 
  and 
  hyomandibular 
  arches 
  have 
  

   also 
  reached 
  a 
  considerably 
  greater 
  development 
  than 
  in 
  the 
  preceding 
  

   IS. 
  Mis. 
  70 
  34 
  

  

  