THE  ORNITHORHYNCHUS  PARADOXUS. 
227 
rhynchus.  But  the  structure  of  the  ovary,  and  that  of  the  ovum  both  before  and  after  it 
has  quitted  the  ovisac,  afford  the  strongest  analogical  proof  of  the  intra-uterine  deve- 
lopment of  the  embryo,  and  at  the  same  time  accord  with  the  ascertained  fact  of  the 
mammary  nourishment  of  the  young  animal ;  there  being  no  store  of  yelk  appended  to 
the  ovum  when  it  quits  the  ovisac,  as  in  the  Bird,  where  it  supplies  the  place  of  a  mam- 
mary secretion  to  the  newly  hatched  chick,  and  where  also  the  voluminous  yelk  and  its 
chalazcB  serve  as  an  essential  nidus  to  the  embryo  at  the  early  stages  of  incubation. 
The  kidneys  were  situated  far  away  from  the  pelvis  and  high  up  in  the  lumbar  region. 
This  marked  deviation  from  the  oviparous  type  is  well  worthy  of  being  taken  into  ac- 
count in  the  consideration  of  the  nature  and  affinities  of  the  Monotremata.  It  is  charac- 
teristic of  the  Mammiferous  type  of  structure,  and  would  seem  to  be  intended  to  give 
free  space  for  the  enlargement  of  the  uterus,  and  to  prevent  the  kidneys  being  affected 
by  the  continued  pressure  of  this  viscus  and  its  contents  during  the  latter  periods  of 
gestation. 
The  situation  of  the  kidneys  with  respect  to  each  other  varied  in  the  two  specimens  : 
in  the  larger  one,  the  left  was  a  little  higher  than  the  right ;  in  the  smaller  one,  it  was 
a  little  lower  :  the  latter  is  the  ordinary  position  in  the  adult.  The  supra-renal  glands 
did  not  correspond  with  this  arrangement ;  but  in  both  instances  the  right  was  higher 
than  the  left,  agreeing  with  the  relative  position  of  the  testes  in  the  male,  and  the  ova- 
ries in  the  female.  In  Mmi,  the  large  size  of  the  supra-renal  glands  is  noted  as  a  foetal 
peculiarity  ;  but  in  the  Ornithorhynchus  they  are  of  minute  size,  their  greatest  diameter 
not  exceeding  J-th  of  a  line  in  the  smaller  specimen  here  described ;  and  they  increase 
in  size  progressively  with  the  growth  of  the  animal,  and  in  a  greater  proportion  than 
the  kidneys  ;  which  increase  would  appear,  therefore,  to  have  relation  to  the  develop- 
ment of  the  generative  organs.    There  were  no  traces  of  the  corpora  Wolffiana. 
The  testes  in  the  small  male  specimen  were  situated  a  little  below  the  kidneys  :  they 
were  of  an  elongated  form,  pointed  at  both  ends,  with  the  epididymis  folded  down,  as  it 
were,  upon  their  anterior  surface.  In  the  female,  the  ovaries  were  freely  suspended  to 
the  loins  in  a  similar  position,  the  right  being  at  this  period  as  large  as  the  left :  it  is 
the  persistence  of  the  latter  at  an  early  stage  of  development  which  occasions  the  dis- 
proportionate size  of  the  two  glands  in  the  adult.  The  still  greater  inequality  of  size  in 
the  oviducts  of  the  Bird  arises  from  a  similar  arrest  of  the  development  of  the  one  on 
the  right  side;  but  both  are  equal  at  an  early  stage  of  existence.  The  uteri  were  straight 
linear  tubes,  scarcely  exceeding  the  size  of  the  ovarian  ligaments. 
The  lungs  were  found  amply  developed  in  both  specimens  ;  the  air-cells  remarkably 
obvious,  so  as  to  give  a  reticulate  appearance  to  the  surface,  and  a  resemblance  to 
the  lungs  of  a  Turtle.  They  had  evidently  been  permeated  by  air  in  the  smaller  spe- 
cimen. 
The  heart  in  both  specimens  was  of  the  adult  form,  with  the  apex  entire  ;  but  the  left 
auricle  was  proportionately  larger  than  in  the  adult  heart,  which  is  correctly  figured  by 
VOL.  I.  2  H 
