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The vole (Microtus agrestis) is an induced ovulator (Austin, 1957a; Breed, 1967) but the interval between copulation and ovulation is not accurately known. Ovulation occurs 9 to 12 hr after injection of pregnancy urine gonadotrophin (Austin, 1957a) but following natural mating, uncleaved eggs or eggs with one cleavage were found in Fallopian tubes 24 hr later (Austin, 1957b). The present study was undertaken to determine the time between coitus and ovulation and to ascertain the associated histological changes in the ovary. Virgin females were placed in cages with virile males, one pair/cage, and observed for 5 to 30 min, after which the pairs were separated. Three females were killed at various stages between 2 to 4 and 18 to 20 hr post coitum (p.c.). Ovaries were removed, sectioned at 7 \g=m\,stained with haematoxylin and eosin and vesicular follicles and corpora lutea (cl) measured and counted as described by Breed (1969). Between 2 and 8 hr p.c., the discus proligerus of the follicles became increasingly separated from the membrana granulosa, strongly staining eosinophilic material appeared around the dispersing discus cells and a corona radiata around the oocyte (Pl. 1, Figs. 1 and 2). No membrane was visible around the nucleus of the ovum by 2 to 4 hr p.c., and by 6 to 8 hr the nucleus was in telophase. Follicles having these characteristics are not seen in virgin females and, hence, are classified as pre-ovulatory follicles. By 9 hr p.c., one female had seven pre-ovulatory follicles in which the theca interna was especially prominent (PL 1, Fig. 1) and 1st polar bodies were seen adjacent to the ova. The second female had two such follicles, as well as one in which the ovum lay free in the antral cavity surrounded by eosinophilic material which extended through a pore in the follicle and another follicle with an almost extruded ovum. Two cl present had slit-like cavities (PI. 1, Fig. 3) and larger, less darkly stained nuclei nearer the periphery (PI. 1, Fig. 4). The third female had four such cl and no pre-ovulatory follicles. By 16 hr p.c., luteal cells had become eosinophilic and few capillaries and connective tissue cells were visible among them. The cavity in the cl tended to become larger and more spherical as the time elapsed after coitus. A cluster of small cells with pycnotic nuclei usually occurred in the cl (PI. 1, Fig. 5). Rupture points were often not visible. Two types of follicles, not observed at earlier stages, were present 10 to 20 hr

OVULATION AND ASSOCIATED HISTOLOGICAL CHANGES IN THE OVARY FOLLOWING COITUS IN THE VOLE (MICROTUS AGRESTIS)