

THE DEVELOPMENT OF THE MUSCULAR SYSTEM. 



281 



heart muscle cells. These may or may not represent the original cells or 

 myoblasts. At all events the muscle fibrils are continuous across the lines. 

 The nuclei proliferate in the syncytium but remain in the central part of the 

 bands or cells, instead of migrating to the periphery as in striated voluntary 

 muscle. 



While the cells are still loosely arranged, rows of granules appear in the 

 cytoplasm, and the granules in each row unite to form a fibril (Fig. 241). The 



fibrils are at first confined to individual 

 cell areas, but as the cells come closer 

 together to form the compact syncytium, 

 they extend through several cell areas 

 and run in different directions (Fig. 242). 

 As development proceeds the fibrils be- 

 come more nearly parallel (Fig. 243). 

 They are first formed in the peripheries 

 of the cells, but later also in the interior, 

 except in a small area immediately sur- 

 rounding the nucleus, where a small 

 amount of undifferentiated cytoplasm 

 remains. The latter is continuous 

 with the cytoplasm or sarcoplasm 

 among the fibrils. As in voluntary 

 seriated muscle the fibrils become differ- 

 entiated into two distinct substances 

 which alternate with each other, thus 

 producing the transverse striation. 



IG. 242. From a section of developing 

 heart muscle in a rabbit embryo of 9 mm. 

 Godlewski. 

 The cells form a compact syncytium. 



Histogenesis of Smooth Muscle. 



The mesenchymal cells which are destined to produce smooth muscle cells 

 are not grouped into any particular primitive structures like the mesodermic 

 somites. They are simply scattered through the general mass of mesenchymal 

 tissue and, like other mesenchymal cells, possess irregular branching forms and 

 distinct spherical nuclei. The internal changes by which these cells are con- 

 verted into muscle cells are not well known. The contractile elements 

 the fibrillae probably represent highly modified portions of the original cyto- 

 plasm but the manner in which the cytoplasm is transformed into fibrillae has 

 not been determined. The external changes consist essentially in an elonga- 

 tion of the irregular mesenchymal cells. The result of this elongation is usually 

 a spindle-shaped cell, but exceptionally cells forked at one or both ends are 

 produced. The original spherical nucleus also shares in the elongation and 

 becomes rod-shaped. 



