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Wohlfart believes that he can distinguish in fetal skeletal muscles a hitherto undescribed type of muscle fibers. These fibers are thicker and longer than the others and are designated as b fibers. This distinction as to size is not to be confused with the distinction between white and red or light and dark fibers.
In the human fetus of 3 months the b fibers cannot with certainty be distinguished from all the others, which are designated as a fibers. The b fibers develop faster and can be distinguished by their size in older fetuses and in the newborn. In the course of childhood the a and b fibers become more and more alike, until they can no longer be distinguished.
In the sartorius muscle of a 20 cm. human fetus the b fibers represent 4.5 per cent of the total number as counted in a cross section; in the newborn
Ueber das Vorkommen verschiedener Arten von Muskelfasern in der Skelettmuskulatur des Menschen und einiger Säugetiere.. Arch NeurPsych. 1938;39(6):1374. doi:10.1001/archneurpsyc.1938.02270060266022