Biopsies from various skeletal muscles were taken post mortem from infants with conceptional ages between 25 and 40 weeks. Cryostat sections were analysed quantitatively. The growth of the mean fibre diameter follows an exponential curve, which is mainly due to an initial predominance of type II-fibres. Before the 34th week of gestation only isolated large "Wohlfart"-fibres show type I staining characteristics. A great number of smaller type I-fibres appear suddenly around the 34th week. Their percentage of the total fibre population increases linearly to 40% at term. The development of the "tonic" type I-fibres correlates with certain aspects of gross motor development. The importance of normal data in the diagnosis of certain congenital myopathies is emphasized.

Biopsies from various skeletal muscles were taken post mortem from 13 infants with conceptional ages between 25 and 40 weeks. Cryostat sections were quantitatively analysed.The growth of the mean fibre diameter follows an exponential curve mainly representing the growth of type 11-fibres which initially predominate.Bcfore the 34th week only isolated large Wohlfart-fibres show type I staining characteristics. A greater numcentage of the total fibre populatioll increases linearly to 40 % at term. I t is suggested that Wohlfart-fibres are a physiologically distinct entity and that typical type I-fibres do not occur before the 34th week. Comparison of muscle groups showed larger fibres in proximal (versus distal) muscles, in cranial (versus caudal) muscles, and in extensors versus flexors. These differences become more pronounced with increasing age. N o differences were found relating to muscle fibre types. ber of smaller type I-fibres appear sud-. -1 Present Adress: Dept. of Paedintrics. Mantilicinl Medidenly around the 34th week. Their per-C~I