It is beyond the scope of this book to describe fully the reproductive physiology of the rat, and the interested reader is referred to the many excellent books on the subject (2-5). Just a few important points ar represented here. Rats reach sexual maturity by the age of 8 wk. Sex in immature rats is differentiated by observation of the perineum. Male rats have a greater distance between the anus and the genital papilla compared to female rats. The gestation period of the rat is 2223 d. The abdomen of a pregnant rat becomes obviously enlarged only during the final week of pregnancy. Under normal mating conditions, the litter size of the rat is about 12. From our laboratory observations, the rat normally gives birth during the day.
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