Fifty-six fetuses and 33 lambs were obtained from a flock of ewes at set gestational intervals between 50 to 180 days after conception. The fetuses and lambs were killed, disected and the sizes and weights of a wide range of skeletal and soft tissues were measured. Five morphological parameters emerged as most suitable for the determination of normal foetal developmental age. By plotting the mean value and ninety-five per cent tolerance limits, the rates of growth and the variability of each parameter were studied. Crown-anus length is useful for determining fetal developmental age from 50 to 100 days gestation; brain weight, long bone length and the number of appendicular ossification centres can be used to determine fetal development age from 50 days gestation until term.
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