The  Physiology  of  Stock-Breeding. 
81 
Experiments  upon  various  animals  (rabbits,  dogs,  &c.),  by 
Fraenckel,^  Dr.  Jolly,  and  myself,^  and  other  investigators  have 
shown  that  if  the  ovaries  with  their  contained  corpora  lutea  are 
extirpated  in  the  earlier  stages  of  pregnancy,  abortion  occurs. 
If  a sufl&cient  quantity  of  luteal  tissue  is  left  behind,  one  or 
more  of  the  developing  embryos  may  escape  abortion.  It  is 
assumed  therefore  that  the  corpora  lutea  contribute  an  essential 
factor  in  the  nutrition  of  the  embryo,  and  are  probably  necessary 
for  those  hypertrophic  changes  whereby  the  wall  of  the  uterus 
prepares  itself  for  the  reception  of  the  fertilised  ovum. 
Since  corpora  lutea  are  derived  only  from  discharged 
follicles,  the  amount  of  luteal  tissue  present  in  the  ovaries  must 
depend  upon  the  number  of  follicles  which  ruptured  at  ovula- 
tion. And  since  the  presence  of  luteal  tissue  is  a necessary 
factor  in  the  development  of  the  fertilised  ovum,  it  would  seem 
to  follow  that  there  is  a causal  relation  between  the  number  of 
follicles  that  discharge  at  ovulation,  and  the  provision  made  by 
the  uterus  for  the  developing  embryos.  A statistical  investiga- 
tion upon  fertility  in  Scottish  sheep  showed  that  among  flocks 
which  were  submitted  to  flushing  or  extra  feeding  at  tupping 
time,  not  only  was  the  fertility  greater,  but  the  frequency  of 
aboi’tion  was  less.  The  latter  result  may  have  depended  upon 
this  correlation  between  the  ovarian  and  uterine  functions, 
since  the  stimulation  of  the  ovarian  metabolism  at  tupping 
time  must  have  resulted  in  the  formation  of  a greater 
quantity  of  functional  luteal  tissue. 
It  is,  of  course,  undeniable  that  sporadic  (as  distinguished 
from  contagious  or  epizootic)  abortion  is  often  induced  by 
accidental  causes  such  as  physical  strain  or  sudden  fright,  but 
it  is  equally  true  that  some  individuals  are  more  liable  to  be 
affected  than  others.  Some  fortuitous  circumstance  may  be  the 
immediate  cause  of  an  animal  “ slipping  ” young,  but  it  is  no 
explanation  of  the  tendency  towards  abortion,  a tendency  which 
may  be  greater  at  some  times  than  others.  The  physiological 
conditions  which  favour  abortion  are  still  only  imperfectly 
understood,  but  one  fact  may  be  regarded  as  established,  namely, 
that  the  state  of  pregnancy  depends  not  only  for  its  initiation, 
but  also  for  its  continuance  (at  least  in  the  earlier  stages)  upon 
the  functional  activity  of  the  ovaries. 
As  already  stated,  flushing  sheep  or  giving  them  additional 
feeding  during  the  tupping  season  is  a means  of  increasing  the 
fertility  of  a flock  at  lambing  time.  So  also  it  has  been  found 
that  the  same  process  hastens  forward  the  time  of  tupping.  It 
* Fraenckel  : “ Die  Function  des  Corpus  Luteum,”  Arch,  of  Gyndh., 
Vol.  68,  1903. 
’ Marshall  and  Jolly  : “ Contributions  to  the  Physiology  of  Mammalian 
Reproduction,”  Phil.  Tram.  Royal  Soc.,  Vol.  198,  B.  1905. 
VOL.  70. 
G 
