290 PROFESSOR J. COSSAR EWART. 



weeks of gestation, it is necessary to ascertain as accurately as possible when 

 fertilisation of the ovum actually takes place. This implies, amongst other things' 

 that each mare used should be kept long enough under observation to admit of the 

 normal length of the cestrous period being determined. 



In a wild Prjevalsky mare {Equus prjewalskii) imported as a yearling from 

 Mongolia, oestrus (the period of desire), first noticed during her third summer, lasted 

 as a rule G or 7 days. Between the end of one period and the beginning of the next 

 there was an interval of 14 or 15 days; i.e. in the wild, as in most domestic mares, 

 the cestrous and anoestrous periods together as a rule occupied 21 days. The period 

 of gestation in the wild mare, if one may judge from the Woburn and other records, 

 averages 357 days or 17 dioestrous cycles of 21 days each. In the wild mare I had 

 under observation oestrus set in 8 days after the birth of her first foal. She was 

 served on the tenth and eleventh days after foaling, and the "period of desire" 

 passed off before the morning of the twelfth day and never recurred u'ntil after her 

 second foal was born. 



In domesticated mares living under nearly natural conditions the length of the 

 oestrous period varies considerably ; and one occasionally hears of in-foal mares 

 coming regularly " in season " during at least the first three months of the gestation 

 period.* I have heard of a mare which proved in foal though the period of desire 

 lasted under 24 hours, f and have had mares under observation which were continu- 

 ously " in season " for 12 or more days. 



In making a collection of embryos with a view to working out the development 

 of the horse it would be a great advantage if one had definite information as to 

 when ovulation takes place, how long the ovum, after escaping from the follicle, 

 retains the power of effectively combining with a sperm, and also how long sperms 

 retain their fertilising power after reaching the oviduct. Unfortunately, we have no 

 definite information, either about ovulation in the Equidse or about the vitality of 

 their ova and spermatozoa. But from my records made since 1895 it may, I think, 

 be inferred (l) that there is in the mare an intimate relation between ovulation and 

 the disappearance of oestrus — that in fact the period of desire as a rule terminates 

 from 10 to 26 hours after ovulation ; (2) that spermatozoa are usually stale 3 or 4 

 days after reaching the oviduct ; and (3) that an ovum usually fails to develop unless 

 it is fertilised within two days after escaping from its follicle. In support of these 

 conclusions, it may be mentioned that when oestrus lasts from the seventh to the 

 tenth day after foaling, the mare is likely to become again pregnant if served on the 



* Mr C. M. Douglas of Auchlochan informs me that Shetland pony mares sometimes take the horse regularly 

 all through the period of gestation and yet produce a normal foal to the first service. Further, I am informed that in 

 both Shetland and Clydesdale fillies oestrus may occur once and again during the earlier months of pregnancy without 

 interfering with the normal development of the foal ; and I have heard of a Clydesdale mare that came in season ami 

 was served three weeks before giving birth to a fully developed hut dead foal. 



f This mare belonged to a herd in the possession of the late Lord Arthur Cecil, a very competent and trust- 

 worthy observer. 



