STUDIES ON THE DEVELOPMENT OF THE HORSE. 291 



eighth, ninth, or tenth clay, but may fail to prove in foal if served on the seventh or 

 not until the eleventh day. Service on the seventh day is in some cases of no avail, 

 because the spermatozoa lose their fertilising power before the ovum escapes from 

 its follicle on the tenth day, while service on the eleventh day is in some cases with- 

 out effect, because the ovum has become stale before it is reached by the spermato- 

 zoon.* 1 With a view to securing a 21 -days embryo, five mares were kept under 

 observation for several months. One of the five, a 13'2-hands Highland pony, came 

 regularly " in season " every third week, and, as in the wild mare, oestrus lasted from 

 6 to 7 days. This Highland mare was served by a 14-hands Arab stallion at 2 p.m- 

 on May 18, 1897, 7 days after she came "in season" (i.e. on the seventh day of 

 cestrus). As she refused to take the horse on May 19, it may be assumed that an 

 ovum escaped and was fertilised soon after the one and only service at 2 p.m. on 

 May 18. On June 8, at 4 p.m. (21 days and 2 hours after service), the mare was 

 killed and the pear-shaped 50-mm. blastocyst represented in fig. 5 was found in the 

 left uterine horn (fig. 15). Obviously the age of the embryo found in the Highland 

 mare may be under, but it cannot possibly be more than two hours over, 21 days. 



Bonnet's 13-mm. blastocyst was taken from a mare 21 days after the first service. 

 How often the mare was served is not stated. If served on the first day of 

 cestrus, and again on, say, the third day, and if, as is quite possible, ovulation took 

 place on the sixth or seventh clay of cestrus, the age of the blastocyst (though 

 removed 21 days after the first service) would only be 14 or 15 days. I am hence 

 inclined to believe that the difference in size between Bonnet's so-called 21 -clays 

 blastocyst (fig. 3) and the blastocyst from the Highland mare (fig. 5) was not due to 

 either arrested development or arrested growth, but to a difference in age — to the 

 ovum from which Bonnet's 13-mm. blastocyst was developed being fertilised 6 or 7 

 days after the first service. 



Martin's 35-mm. blastocyst was obtained from an eight-year-old mare, which had 

 a foal the previous year. This implies that cestrus probably lasted 6 or 7 days. The 

 mare was served at 1.30 p.m. on April 2, and killed at 8 p.m. on April 23. When 

 cestrus set in and passed off is not mentioned. If ovulation took place on April 2 

 a few hours after service, the age of Martin's embryo was in all probability 21 clays ; 

 but if the ovum only escaped from its follicle on, say, April 6, the age would only be 

 17 clays. The difference in size and number of somites between Martin's so-called 

 21 -days embryo (fig. 7) and my 21 -days embryo (figs. 8 to 11) is considerable. Seeing 

 that no information is given by Martin as to whether the mare which yielded his 

 embryo was served at the middle or end of oestrus, I think it may be assumed that - 

 the difference between the embryos represented in figs. 7 and 8 is more likely to be 

 due to a difference in age than to either arrested growth or arrested development. 

 Taking these and other facts into consideration, I shall, I think, be justified in 



* In all probability service sometimes induces ovulation ; but, as a rule, no matter how often the mare is served, 

 the follicle remains intact until the seventh or eighth day of oestrus, 



