Musculature

The precursors of limb muscle cells do not originate from the parietal mesoderm of the lateral plate, but instead migrate from the ventral part of the dermomyotomes along extracellular matrix structures. They are still capable of division and form two pre-muscular primordia: one for the future extensors dorsally, and one for the flexors ventrally. The pre-muscular primordia are initially subdivided into layers, and then into individual muscle blastemas. Some individual blastemas fuse together again to form definitive muscle primordia whereas others degenerate.
The incoming myoblasts exhibit high mitotic activity. Postmitotic myoblasts fuse together to form multinuclear myotubes. Finally, the nuclei in the myotubes are shifted towards the periphery of the muscle fibre. In this way, the definitive skeletal muscle fibre is formed. Muscles are unisegmental if they originate from only one myotome. Multisegmental muscles develop from several myotomes. Some muscle primordia are shifted secondarily towards the skeleton of the trunk (spinoscapular, spinohumeral and spinocostal muscles). An example of a spinohumeral muscle is the latissimus dorsi muscle.
The first elementary movements of the limbs start at week 7. Combined movements of the limbs, thorax and head begin between weeks 12-16. From week 16, the mother can feel her baby move (quickening).