Ovary and reproductive tract
At birth, the ovary contains all the germ cells that the animal will ever need: these act as a reservoir from which the primary follicles develop. At the onset of puberty or sexual maturity, several primary follicles develop to form ripe Graafian follicles.
In multiparous species, there will be many follicles, divided between the two ovaries, but not necessarily equally. Each Graalian follicle consists of an ovum formed by the process of meiosis and containing the haploid number of chromosomes (see Ch. 11. This is suspended in fluid and surrounded by an outer layer of follicular cells. The Graalian follicle secretes the hormone oestrogen (Fig. 11.8).When the follicle has reached full size, it ruptures to release the ovum - the process of ovulation. The ovum passes down the uterine tube and the remaining follicular tissue becomes reorganised to form the corpus luteum. The corpus Iuleum secretes the hormone progesterone.
Within the reproductive tract, the uterine walls become thickened and more glandular to create a suitable environment for implantation of the fertilised ova. The vaginal epithelium also changes and the blood-stained discharge seen in proestrus in the bitch comes from the lining of the vagina. Study of the
Cpitliehiil cells in vaginal smears at intervals during proestrus can be used to gauge the correct time for mating - this technique is known as exfoliative ιi∣tol(H∣ι∣.