THE HISTOGENESIS OF MUSCLE 



2 9 I 



nective tissue (McGill, 1907). The myoglia fibrils may extend from cell 

 to cell, thus connecting them. These fibrils are the products of coalesced 

 granules found within the cytoplasm of the myoblasts. In embryos of 

 30 mm. fine myofibrillae are differentiated in the cytoplasm of the myoblasts 

 and give it a longitudinally striated appearance. The cytoplasmic pro- 

 cesses of the muscle cells, the cytoplasmic bridges, later give rise to white 



FIG. 297. Two stages in the development of smooth muscle fibers (after McGill). A, 

 from the esophagus of a 13 mm. pig (X55o); 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. 



connective-tissue fibers which envelop the muscle fibers and bind them 

 together. Smooth muscle also increases in amount by mitotic division 

 and the transformation of interstitial connective -tissue cells. 



Striated Skeletal Muscle. All striated voluntary muscle is derived 

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



FIG. 298. Stages in the histogenesis of skeletal muscle (after Godlewski). A, myoblast 

 a 13 mm. sheep embryo; B, myogeneous myofibrils in a myoblast of a 10 mm. guinea pig em- 

 bryo; C, myoblast with longitudinally splitting striated myofibrils from an 8.5 mm. rabbit 

 embryo. 



the trunk) of from the mesenchyma (muscles of the head). According 

 to Bardeen (1910), after the formation of the sclerotome (Fig. 290), 

 which gives rise to skeletal tissue, the remaining portion of the primitive 

 segment constitutes the myotome; all of its cells become myoblasts. On 

 the contrary Williams (1910), finds that in the chick only the cells of the 

 dorsal and mesial walls of a mesodermal segment are myoblasts. By 



