The mares chosen for the investigation of the changes in the reproductive organs during the oestrous cycle were kept under observation for some time before they were killed. The duration of the heat period in these animals was 7 days and the length of the dioestrus was 16 days. Ovulation takes place at about a day before the end of oestrus. The size of the ovary during the oestrous cycle is chiefly influenced by the growing Graafian follicle. The number of follicles present at different stages varies greatly. The numerous small follicles present at the beginning of oestrus disappear later in the cycle; it is suggested that this may be due to the lack of follicle-stimulating hormone. The colour of the corpus luteum varies greatly at different stages of the cycle. The rupture of the follicle is associated with some bleeding. The active stage of the corpus luteum is very short, and the maximum diameter of the corpus luteum seems to be always below that of the Graafian follicle. The greater development of the Graafian follicle, with its secretion of oestrin, in the mare leads to its playing a more important role than in the cow and the sow, in which species the corpus luteum takes a m ore dominant part in the cycle. It appears that the much longer oestrus in the mare than in the cow is due to the longer time required by the follicle to come to the surface and to break through. This is probably due to the peculiar structure of the ovary in the mare, since the ovulation, which is spontaneous, can only occur in the small ovulation fossa. No pronounced secretion stage occurs during oestrus in the Fallopian tubes.
Anatomical and histological changes during the oestrous cycle in the mare