292 



HISTOGENESIS 



and give it a longitudinally striated appearance. The cytoplasmic processes of 

 the muscle cells, the cytoplasmic bridges, later give rise to white connective 

 tissue fibers wMch envelop the muscle fibers and bind them together. Smooth 

 muscle increases in amount: (1) by the formation of new fibers from the mesen- 

 chyme of the embryo; (2) by the transformation into muscle fibers of interstitial 









%rm- 



Fig. 297. — Two stages in the development of smooth muscle fibers (after McGiU). A, from the 

 esophagus of a 13 mm. pig (X 550); coalescing granules give rise to coarse myoglia fibrils. B, from the 

 esophagus of a 27 mm. pig (X 850); both coarse myoglia fibrils and fine myofibrils are present. 



cells; (3) by the multipUcation of their nuclei by mitosis in the more advanced 

 fetal stages. 



Striated Skeletal Muscle. — All striated voluntary muscle is derived from 

 the mesoderm, either from the myotomes of the segments (muscles of the trunk) 

 or from the mesenchyma (muscles of the head). According to Bardeen (in 

 Keibel and Mall, vol. 1), after the formation of the sclerotome (Fig. 290), which 



Fig. 298. — Stages in the histogenesis of skeletal muscle (after Godlewski). A, from 13 mm. sheep 

 embryo; B, homogeneous myofibrils in myoblast from 10 mm. guinea pig embryo; C, myoblast from 8.5 

 mm. rabbit embryo with longitudinally splitting striated myofibrils. 



gives rise to skeletal tissue, the remaining portion of the primitive segment con- 

 stitutes the myotome. All the cells of the myotome give rise to myoblasts. Wil- 

 liams (Amer. Jour. Anat., vol. 11, 1910), working on the mesodermal segments of 

 the chick, finds that only the dorsal and mesial cells are myoblasts. By multi- 

 plication they form a mesial myotome, while the lateral cells of the orio-inal 



