THE HISTOGENESIS OF MUSCLE 299 



and their cytoplasm becomes more abundant. The resulting spindle-shaped cells 

 (Fig. 289 A) remain attached to each other by cytoplasmic bridges and develop 

 in the superficial layer of their cytoplasm course myoglia fibers (Fig. 289 B) 

 similar to the primitive fibrillas of connective tissue. The myoglia fibers may 

 extend from cell to cell, thus connecting them. These fibers 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 processes of 

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

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

 muscle increases in amount: (i) by the formation of new fibers from the 

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

 interstitial cells ; (3) by the multiplication of their nuclei by mitosis in the more 

 advanced fetal stages. 



Striated Cardiac Muscle. This is developed from the splanchnic mesoderm 

 which forms both the epicardium and the myocardium. The cells of the myo- 

 cardium at first form a syncytium in which myofibrillas develop from chondri- 

 oconta or cytoplasmic granules. The myofibrillae are developed at the periphery 

 of the syncytial strands of cytoplasm and extend long distances in the syncytium. 

 They multiply rapidly in number and become differentiated each into alternating 

 dark and light bands, due to a difference in density. The syncytial character of 

 cardiac muscle persists in the adult and the nuclei remain central in position. The 

 intercalated discs typical of adult cardiac muscle appear relatively late, just 

 before birth in the guinea-pig, according to Jordan and Steele. 



Striated Voluntary 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. i), after the formation of the sclerotome (Fig. 282 A), which 

 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. 88), 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 original 

 mesodermal segment persist as a dermatome and give rise only to the connective 

 tissue of the dermis (Fig. 291). The dermatome lies lateral to the myotome 

 and the two together constitute the dermo-myotome, according to Williams (Fig. 

 45). 



