80 
The  Physiology  of  Stock-Breeding . 
characterise  the  uterus  during  the  earlier  part  of  pregnancy  is 
associated  with  the  formation  of  bodies  called  corpora  lutea  in 
the  ovaries.  For  after  ovulation  and  when  the  ova  have  been 
discharged  the  follicular  epithelial  cells,  instead  of  degenerat- 
ing, undergo  an  enormous  increase  in  size  and  become  converted 
into  luteal  cells,  so  called  because  they  contain  a yellow  pigmen- 
ted substance  known  as  lutein.  These  luteal  cells  fill  up  the 
cavity  of  the  discharged  follicle  which  is  thus  converted  into 
Fig.  5. — Corpus  luteum  or  discharged  follicle  in  ovary  after  transplantation  into 
kidney. 
I.C.,  luteal  cells  filling  cavity  ; c.t.,  connective  tissue  wall  of  foDicle  ; k.t.,  tissue 
of  kidney. 
a corpus  luteum.  If  pregnancj"  supervenes  as  a result  of  con- 
ception during  oestrus,  these  corpora  lutea  continue  to  grow 
until  nearly  half  way  through  that  period,  when  they  gradually 
degenerate,  but  if  the  animal  fails  to  become  pregnant  they  go 
on  increasing  in  size  for  only  a few  days  and  then  dwindle. 
The  fully  formed  corpus  luteum  is  provided  with  a rich  blood 
supply,  and  has  all  the.  appearance  of  an  internally  secreting 
gland. 
