The  Physiology  of  Stock-Breeding. 
69 
The  spermatozoa  are  pi’oduced  in  enormous  quantities  in 
the  testes,  and  in  most  of  the  domestic  animals  the  process  of 
formation  appears  to  go  on  almost  continuously.  The  produc- 
tion of  ova,  on  the  other  hand,  is  limited,  both  in  regard  to  the 
periods  of  ripening  and  the  number  that  are  matured.  For  in 
the  female  ovulation,  or  the  discharge  of  ripe  ova  from  the 
ovaries,  is  restricted  to  the  periods  of  oestrus,  and  even  at  these 
times  the  number  of  ripe  ova  produced  is,  on  an  average, 
scarcely,  if  at  all,  greater  than  the  normal  number  of  young 
born  in  a litter. 
The  complete  oestrous  (or  female  generative)  cycle  consists 
of  periods  of  rest  alternating  with  periods  of  activity  or  “ heat” 
periods.  The  latter,  as  was  first  pointed  out  by  Heape,^  consist 
of  two  sub-periods.  The  first  of  these  is  the  pro-oestrum,  during 
which  the  generative  organs  become  congested,  and  external 
bleeding  associated  with  a discharge  of  mucus  from  the  genital 
apertures  sometimes  takes  place  (notably  in  dogs).  This  gener- 
ally lasts  for  only  a day  or  two,  but  in  dogs  may  extend  for  a 
week  or  ten  days.  It  must  be  regarded  as  an  act  of  preparation 
on  the  part  of  the  generative  organs,  and  more  particularly  the 
uterus,  for  the  reception  of  a fertilised  ovum,  for  at  this  period 
the  superficial  tissues  of  the  uterine  mucous  membrane  (which 
lines  the  cavity)  undergo  a process  of  renewal.^  The  pro-oes- 
trum is  followed  by  the  oestrus  (or  second  sub-period)  which 
may  be  regarded  as  the  “heat”  period  proper.  It  is  then  only 
that  the  female  is  normally  ready  to  receive  the  male,  and 
ovulation  takes  place.  The  duration  of  the  oestrus  is  somewhat 
variable,  a mare  occasionally  remaining  on  “heat”  for  as  much 
as  a week,  but  this  is  unusual.  One  or  two  days  is  a more 
ordinary  time,  but  a ewe  may  sometimes  be  in  a condition  to 
receive  the  tup  for  only  a few  hours.  It  is  important  to  remem- 
ber that  ovulation  cannot  as  a rule  occur  at  other  times  than  the 
oestrous  periods,  and  that  sometimes  it  may  be  delayed  until 
near  the  end  of  the  oestrus.  Consequently  a too  early  service 
during  heat  may  result  in  barrenness,  simply  because  at  the  time 
when  the  female  received  the  spermatozoa  there  were  no 
discharged  ova  for  them  to  fertilise. 
The  number  of  ova  which  are  discharged  at  a single  oestrus,  as 
already  remarked,  appears  to  bear  a close  relation  to  the  normal 
number  of  young  produced  at  a time.  In  the  cow,  so  far  as  I 
^ Heape  : “The  Sexual  Season,”  &c.,  Quart.  Jour.  Micr.  Vol.  44, 
1900.  See  also  Marshall : “ The  CEstrous  Cycle,  &c.,  in  the  Sheep,”  Phil.  Tram. 
Royal  Soc.,  B,  Vol.  196,  1903. 
The  pro-oestrous  changes  may  however  be  so  slight  as  to  be  unrecognisable 
externally,  and  yet  a normal  oestrus  may  supervene.  Owing  to  this  some 
“cowmen”  will  let  a “period”  pass  by  and  so  lose  three  weeks,  and  possibly 
the  chance  of  getting  an  animal  in  calf  for  that  season,  simply  because  they 
failed  to  detect  any  pro-oestrous  discharge. 
