﻿45G 
  REPORT 
  OF 
  COMMISSIONER 
  OF 
  FISH 
  AND 
  FISHERIES. 
  [30] 
  

  

  ill 
  the 
  upper 
  jaw 
  in 
  one 
  case 
  are 
  double; 
  that 
  is, 
  are 
  formed 
  of 
  two 
  pri- 
  

   mary 
  simple 
  cusps 
  of 
  the 
  haplodont 
  form 
  fused 
  together. 
  The 
  spaces 
  

   between 
  these 
  double 
  or 
  bicuspid 
  germs 
  is 
  greater 
  in 
  two 
  instances 
  than 
  

   between 
  the 
  simple 
  haplodont 
  germs. 
  This 
  raises 
  an 
  interesting 
  ques- 
  

   tion 
  as 
  to 
  the 
  genesis 
  of 
  bicuspid 
  and 
  two-rooted 
  teeth. 
  Whether, 
  in- 
  

   deed, 
  teeth 
  of 
  the 
  bicuspid 
  type 
  have 
  not 
  aris*en 
  in 
  some 
  cases 
  by 
  the 
  

   fusion 
  or 
  concrescence 
  of 
  two 
  primitively 
  distinct 
  haplodont 
  or 
  unicus- 
  

   pid 
  germs. 
  Such 
  a 
  mode 
  of 
  origin 
  of 
  certain 
  types 
  of 
  teeth 
  from 
  a 
  more 
  

   numerous 
  unicuspid 
  series 
  of 
  germs 
  is, 
  at 
  least, 
  worthy 
  of 
  serious 
  con- 
  

   sideration. 
  

  

  In 
  the 
  foetus 
  of 
  Globiocephalus 
  (Fig. 
  9, 
  Plate 
  I) 
  the 
  tooth 
  germs 
  w 
  T 
  ere 
  

   not 
  yet 
  distinguishable 
  in 
  sections 
  as 
  more 
  than 
  pronounced 
  thickenings 
  

   of 
  the 
  oral 
  epidermis 
  at 
  the 
  point 
  where 
  the 
  dental 
  furrow 
  would 
  appear 
  

   later. 
  No 
  traces 
  of 
  the 
  enamel 
  organ 
  or 
  of 
  the 
  dentinal 
  elements 
  of 
  the 
  

   teeth 
  could 
  be 
  made 
  out. 
  

  

  At 
  a 
  similar 
  stage 
  the 
  mammary 
  glands 
  appear 
  as 
  simple 
  thickenings 
  

   of 
  the 
  epidermis 
  on 
  either 
  side 
  of 
  the 
  genital 
  opening 
  of 
  the 
  female 
  

   foetus 
  of 
  Globiocephalus. 
  There 
  are 
  as 
  yet 
  only 
  the 
  faintest 
  traces 
  of 
  the 
  

   mammary 
  fossae 
  shown 
  in 
  the 
  sections. 
  The 
  rudiment 
  of 
  the 
  gland 
  is 
  

   a 
  solid 
  pyriform 
  mass 
  of 
  cells 
  which 
  is 
  thrust 
  inwards 
  from 
  the 
  epider- 
  

   mis 
  into 
  the 
  mesoblast. 
  

  

  The 
  brain 
  is 
  quite 
  smooth 
  in 
  the 
  foetus 
  represented 
  in 
  Fig. 
  9, 
  Plate 
  I, 
  as 
  

   in 
  the 
  embryos 
  of 
  other 
  mammals 
  of 
  the 
  same 
  relative 
  stage 
  of 
  advance- 
  

   ment. 
  There 
  is 
  a 
  very 
  pronounced 
  cranial 
  flexure, 
  nearly 
  or 
  quite 
  as 
  

   great 
  as 
  in 
  a 
  human 
  foetus 
  of 
  the 
  same 
  stage 
  of 
  advancement. 
  This 
  

   flexure 
  also 
  involves 
  the 
  brain. 
  I 
  find 
  no 
  evidence 
  of 
  the 
  existence 
  of 
  

   olfactory 
  lobes 
  such 
  as 
  arc 
  so 
  well 
  developed 
  in 
  the 
  brains 
  of 
  foetal 
  Ro- 
  

   dents. 
  The 
  cerebral 
  vesicles 
  are 
  quite 
  thin-walled 
  and 
  smooth, 
  so 
  that 
  

   the 
  lateral 
  ventricles 
  are 
  spacious. 
  The 
  cerebral 
  vesicles 
  are 
  also 
  de- 
  

   pressed, 
  and 
  reflected 
  back 
  over 
  the 
  mid-brain 
  to 
  some 
  extent. 
  

  

  III. 
  — 
  The 
  hypertrophy 
  and 
  differentiation 
  of 
  the 
  caudal 
  

  

  VERTEBRiE 
  OF 
  CETACEANS 
  INTO 
  TWO 
  SERIES. 
  

  

  One 
  of 
  the 
  most 
  remarkable 
  traits 
  of 
  the 
  cetaceans 
  is 
  the 
  differentia- 
  

   tion 
  which 
  their 
  caudal 
  vertebra 
  1 
  , 
  have 
  undergone. 
  One 
  may 
  divide 
  

   these 
  into 
  two 
  groups, 
  viz, 
  (1) 
  those 
  caudal 
  vertebrre 
  intervening 
  be- 
  

   tween 
  the 
  last 
  of 
  the 
  lunibars 
  and 
  the 
  first 
  one 
  in 
  front 
  of 
  the 
  flukes, 
  

   and 
  (2) 
  those 
  terminating 
  the 
  vertebral 
  column 
  and 
  lying 
  between 
  the 
  

   bases 
  of 
  the 
  flukes. 
  The 
  first 
  group 
  is 
  characterized 
  by 
  the 
  remarkably 
  

   uniform 
  vertical 
  diameter 
  and 
  length 
  of 
  their 
  centra 
  and 
  well-developed 
  

   chevron 
  bones; 
  the 
  second, 
  on 
  the 
  other 
  hand, 
  have 
  lost 
  all 
  but 
  the 
  

   merest 
  traces 
  of 
  processes, 
  and 
  rapidly 
  diminish 
  in 
  size 
  so 
  that 
  the 
  last 
  

   centrum 
  may 
  be 
  present 
  only 
  as 
  a 
  diminutive 
  osseous 
  or 
  cartilaginous 
  

   nodule. 
  

  

  The 
  uniform 
  dimensions 
  of 
  the 
  centra 
  of 
  the 
  first 
  or 
  anterior 
  group 
  

   indicates 
  that 
  the 
  posterior 
  ones 
  of 
  that 
  series 
  at 
  least 
  have 
  been 
  hy- 
  

  

  