STUDIES ON THE DEVELOPMENT OP THE HORSE. 295 



of eventually proving fertile even if she several times " broke service." Some of the 

 follicles were under 5 mm., some were over 40 mm. in diameter, but even in a 6-mm. 

 follicle (fig. l) I found what appeared to be a well-advanced ovum (fig. 2). The 

 largest follicle in the left ovary (fig. 15) doubtless contained a nearly ripe ovum. 

 In the mare, up to the end of the sixth week the trophoblast is at the best only 

 loosely connected to the uterine mucous membrane ; it is hence mainly prevented 

 from escaping from the uterine horn by internal hydrostatic pressure.* Had the 

 blastocyst escaped from the uterus (i.e. had the Highland mare aborted, " broken 

 service") during the third week, the large follicle seen in fig. 15 would have dis- 

 charged an ovum on or about June 8, thus giving the mare a chance of again be- 

 coming pregnant without any loss of time. On the other hand, had the gestation 

 in the mare been allowed to run its course, the maturation of ova would have been 

 arrested and the size of the enlarged follicles in both ovaries gradually reduced. 

 That the large follicle in fig. 15 would have remained intact is suggested by its 

 outer wall being decidedly thicker than that of the wall of the large follicle in the 

 ovary (fig. 16) of a non-pregnant mare which died on the second day of oestrus, t 



The corpus luteum in the ovary of the mare which yielded Martin's embryo was 

 of a soft consistency, pale red in colour, and about double the size of a lentil. The 

 corpus luteum found in the left ovary of the Highland mare is represented in fig. 1 8 

 natural size. The corpus luteum seen in fig. 17 occurred in the ovary of a mare 

 which died three months after giving birth to a foal.| 



(2) The Oviduct and Uterus. — In the mare at the end of oestrus the trumpet-like 

 expansion at the free end of the oviduct (fig. 19) lies near to, or in contact with, the 

 follicle about to discharge an ovum. How long the "trumpet" retains this position 

 after ovulation is not known — in the sheep the trumpet is said to adhere to the 

 surface of the ovary for eight or nine days after service. The trumpet is lined with 

 numerous lamellae which converge to the ostium abdominale to become continuous 

 with the interrupted ridges extending along the oviduct ; it will be observed from 

 fig. 19 that the lamellae lining the trumpet are continued for some distance over its 

 outer . surface. The expanded upper part of the oviduct, immediately within the 

 ostium, may play the part of a seminal receptacle — in a mare killed about 24 hours 

 after service it contained numerous active spermatozoa. The oviduct opens into 

 the uterine horn (fig. 19) by a minute aperture surrounded by the terminal portions 

 of the lamellae of the oviduct. 



At birth the mucous membrane of the uterus presents few furrows and only 



* Experiments by Marshall and Jolly seem to show that the corpus luteum provides a secretion essential for 

 the attachment of the embryo and for its nourishment during the first stages of pregnancy. " Contributions to the 

 Physiology of the Mammalian Reproduction," Phil. Trans., Ser. B, vol. cxcviii, 1905. 



f Purther inquiries may show that in the case of mares that come in use during the period of gestation ovula- 

 tion may occasionally take place. 



% The function of the corpus luteum in the mare is dealt with in The Physiology of Reproduction, by F. H. A. 

 Marshall, Longmans, 1910. 



TRANS. ROY. SOC. EDIN., VOL. LI, PART II. (NO. 7). 43 



