Abstract
The largest and smallest littermates were chosen by weight from litters of 38 days' gestation to 1 day post partum. Complete frozen sections of the semitendinosus muscle were used to provide a qualitative and quantitative account of the development of the primary and secondary generations of muscle fibres. The results showed that the time of formation of primary and secondary fibres, and the numbers of primary fibres formed, were the same in both large and small littermates. The number of secondary fibres formed, however, was lower in the smaller fetuses and resulted in there being a 17% difference in total fibre number at birth. Primary fibres in small fetuses were smaller, due to the smaller central myofibril-free region. This small size may have restricted the available surface area for secondary fibre formation. Fibre hyperplasia was found to cease between 85 and 95 days' gestation, and so the fibre number difference is likely to be permanent.
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