PHYSIOLOGY. 31 
this situation, and development of the embryo now 
commences. 
The time which must necessarily elapse between 
the periods of impregnation and parturition has been 
termed the period of gestation. 
The Period of Gestation. 
This period varies very much in different animals, 
and also in the same class of animals when subjected 
to different surroundings and treatment. 
In the Mare, the period of gestation is about 
forty-eight weeks, but varies more in this animal 
perhaps than in any other. Some brood mares foal 
regularly at a little over ten months, while others are 
pregnant nearly a year, and then give birth to a 
healthy foal. 
Note.—Mr. John Renfrew, M.R.C.V.S., Glasgow, 
has furnished me with the record of two cases, one, 
that of a Pony Mare belonging to Mr. Mitchell, 
Polmont, Scotland, which had been served for twelve 
months and three days, and was then said to be in 
season; was sent to the stallion and served, and the 
next day gave birth to a healthy colt foal. 
The other case was that of a thoroughbred Mare, 
the property of Mr. Stevenson, Hurlet, Scotland, 
which carried her foal twelve and a half months 
regularly. 
As a rule, a Mare carrying a colt-foal (horse-colt) 
is pregnant several dayselonger than with a filly-foal 
(mare-colt). If the animal be kept at gentle work, it 
will generally give birth several days earlier, and 
