74 
The  Physiology  of  Stock-Breeding. 
only  what  the  physiologist  would  expect,  for  the  number  of 
lambs  born  must  depend  upon  the  number  of  ova  discharged  at 
periods  of  oestrus.  Again  there  can  be  no  doubt  that  a stimulat- 
ing diet  favours  the  development  of  follicles,  and  so  provides  a 
larger  number  available  for  discharging  the  mature  ova  at  the 
time  of  tupping.  Such  a diet  administered  at  any  other  season 
is  not  so  likely  to  increase  the  fertility,  because  the  condition  of 
the  follicles  in  the  ovaries  is  one  of  immaturity  (excepting  in 
those  varieties  which  can  breed  constantly),  and  unless  the 
“flushing”  is  continued  until  tupping  time  the  number  of  ripe 
follicles  present  at  any  single  heat  period  is  less  likely  to  be 
increased.  Moreover,  there  is  evidence  that  heavy  flushing 
when  succeeded  by  a mere  sustenance  diet  is  conducive  to  a 
degeneration  of  follicles,  and  so  is  liable  to  impair  the  fertility 
of  the  sheep  in  subsequent  years. 
It  has  been  claimed  by  German  veterinarians  that  the  drug 
yohimbine,  if  administered  to  domestic  animals,  often  acts  as  a 
remedy  for  impotence  and  sterility.  That  it  can  cause  a very 
marked  congestion  of  the  entire  generative  tract  in  rodents  has 
been  shown  by  Dr.  Cramer  and  myself.^  More  recently  I have 
obtained  evidence  that  yohimbine  acts  directly  on  the  tissues, 
since  it  can  produce  congestion  of  the  uterus.  Furthermore  it 
appears  to  favour  the  matui’ation  of  follicles  and  to  prevent 
follicular  degeneration,  but  more  evidence  is  required  on  this 
point.  It  would  seem  clear,  however,  that  by  increasing  the 
blood  supply  to  the  ovaries  it  must  provide  these  organs  and 
the  developing  ova  which  they  contain  with  a more  abundant 
nourishment,  and  so  favour  an  increased  fertility.  This  is 
probably  (in  part  at  least)  the  physiological  explanation  of  the 
results  obtained  by  veterinarians  and  others  in  Germany.  At 
the  same  time  there  can  be  no  doubt  that  this  drug  produces  a 
marked  aphrodisiac  action,  that  is  to  say  it  increases  sexual 
desire,  producing  this  effect  both  in  males  and  females. 
Selection  among  Germ  Cells. 
Besides  fertilising  the  ova,  the  spermatozoa  have  the  function 
of  transmitting  the  hereditary  characters  of  the  male  parent. 
In  a similar  way  the  ova  are  the  bearers  of  the  maternal 
characters.  There  are  many  reasons  for  concluding  that  the 
parental  characters  are  not  distributed  equally  among  all  the 
ova  or  spermatozoa,  and  modern  Mendelian  research  has  afforded 
much  evidence  in  favour  of  the  theory  that  there  is  a definite 
segregation  by  which  the  carriers  of  alternate  or  different  char- 
acters pass  into  different  germ  cells.  Heape  and  others  have 
recently  suggested  that  there  is  a struggle  for  existence  among 
1 Cramer  and  Marshall : “ Preliminary  Note  on  the  Action  of  Yohimbine 
on  the  Generative.  System,”  Jour,  of  Economic  Biology ^ Vol.  2,  1908. 
