Abstract
To establish their chronology in rats the somites of 902 embryos were counted. They were the products of 89 pregnancies terminated at three hourly intervals from 231 hours to 288 hours and then six hourly until 324 hours. Somites form at a virtually constant rate of one somite per 1.87 hours. This is incorporated in the regression equation (formula; see text) where y = number of somites, x = hours of gestation from 06.00 on the morning after overnight mating. The greatest variation in the maturity of embryos within any time-determined sample was nine somites (= 16.8 hours) and the least was three somites (= 5.6 hours). Some uses for the data in experimental teratology are suggested.
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