THE DEVELOPMENT OF THE MUSCULAR SYSTEM. 311 



nective tissue, the perimysium and endomysium, the epimysium being derived 

 from the mesenchymal tissue which originally surrounded the muscle. 



THE VISCERAL MUSCULATURE. 



The visceral musculature is derived wholly from the mesoderm, but not 

 from the myotomes. The striated involuntary muscle or heart muscle is de- 

 rived from the mesothelial lining of the ccelom, the smooth muscle from the 

 mesenchymal tissue in various regions of the body. The heart muscle develops 

 only in connection with the heart and consequently occurs in the adult only in 

 that organ. Smooth muscle develops to form integral parts of certain structures 

 such, for example, as the alimentary tube, glands, blood vessels, and skin. 



Histogenesis of Heart Muscle. 



When the simple tubular heart is first formed, the splanchnopleure projects 

 into the ccelom (primitive pericardial cavity) along each side (Fig. 194; also p. 

 222). The mesothelium covering these projections is destined to give rise to 



y^7"r;\ / 



FIG. 279. From a section of developing heart muscle from a rabbit embryo of 9 mm. Godlewski 



a, Cell body with granules arranged in series; b, cell body with centrosome and attraction sphere; 



c, branching fibril; d, fibrils extending through several cells. 



the myocardium. The mesothelial cells which are at first closely packed to- 

 gether with but little intercellular substance, assume irregular branching forms 

 and the branches anastomose freely (Fig. 279). After the cells become loosely 

 arranged, they again become closely packed to form a compact syncytium, in- 

 dividual cells apparently assuming the shape of heavy bands (Fig. 280). Ir- 

 regular transverse bands next appear, dividing the syncytium into the so-called 



