Sexual development in the Mongolian gerbil, Meriones unguiculatus, with particular reference to the ovary.

In the Mongolian gerbil, vaginal opening occurs at 40 to 60 days of age (Nakai, Nimura, Tamura, Shimizu & Nishimura, 1960; Schwentker, 1963; Marston & Chang, 1965), whilst the age of sexual maturity has been given as 9 to 12 weeks (Schwentker, 1963; Marston & Chang, 1965). Recently, Norris & Adams (1972a) estimated that the mean age at first conception was 75 days under the most favourable conditions, namely 60-day-old females paired with sexually experienced males. Normal growth data from birth to 150 days of age have been presented by Norris & Adams (1972b). The present study was undertaken in order to obtain information on ovarian development in relation to puberty and sexual maturity in this species. Recently, Stark (1973) has reported on ovarian development in gerbils aged 0 to 21 days. Observations were made on a total of fifteen female Mongolian gerbils which came from our own colony, details of which have been given elsewhere (Norris & Adams, 1972b). They were selected from litters containing five young. Beginning at 20 days of age, animals were killed with an overdose ofchloroform