Final answer:
Skeletal muscle fibers during embryonic development are derived from myoblasts, which originate from the mesoderm layer and form into multinucleate structures known as myotubes.
Step-by-step explanation:
During the embryonic development stage, skeletal muscle fibers are derived from special embryonic cells known as myoblasts. These myoblasts originate from the mesoderm, which is one of the three germ layers formed during the early phases of embryogenesis. More specifically, paraxial mesodermal cells adjacent to the neural tube form segmented blocks of cells called somites, from which most of the skeletal muscles originate, except those in the head and limbs. Myoblasts, being muscle-forming stem cells, migrate to different regions in the developing embryo and fuse together to form a syncytium or myotube. The fusion of multiple myoblasts, which are each multinucleated, gives rise to the mature skeletal muscle fibers known for their multiple nuclei and striated appearance under a microscope.