HISTOGENESIS OF MUSCLE TISSUES 



703 



coursing anteriad below the foregut (Chap. 17). These two enveloping layers 

 of mesoderm give origin to the epicardiuin and myocardium of the heart, and 

 in consequence they are referred to as the epimyocardial rudiment. From the 

 surfaces of the two layers of hypomeric mesoderm which face the primitive 

 blood capillaries, mesenchymal cells are given off. These mesenchymal cells 

 constitute the myocardial primordium. The outer wall of each hypomeric 

 layer of mesoderm, however, retains its epithelial character and eventually 

 gives origin to the epicardium or coelomic covering of the heart. The mesen- 

 chymal cells which form the myocardial primordium surround the two endo- 

 cardial rudiments (blood capillaries) and later form an aggregate of coalesced 

 cells, i.e., a syncytium. The future heart musculature arises from this syncytium. 



As the mass of the myocardial syncytium increases in size, the nuclei be- 

 come irregularly scattered, and myofibrils make their appearance. The number 

 of myofibrils rapidly increases, and dark bands of anisotropic substance (i.e., 

 substance which is doubly refractive under polarized light) alternate with 

 lighter bands of isotropic substance. Z lines soon appear which bisect the 

 lighter segment of the myofibrils. 



The myofibrils increase, and the myocardial syncytium gradually becomes 

 drawn out into elongated strands of cytoplasm which appear to anastomose 

 (fig. 325C). The nuclei are scattered within these strands. As the myofibrils 



nucle;us 



NTERCALATED DISC 



DARK BAND 

 LIGHT BAND 



MEDIAN MEMBRANE 

 N BANDS OF HENSEN 



GROUND MEMBRANE OR Z LINEOFKRAUSE 



DIAGRAM OF MUSCLE STRIATIONS 



Fig. 325. Structure of the three types of muscle tissue. (All figures redrawn from 

 Bremer (1936), Textbook of Histology, Philadelphia, Blakiston.) (A) Skeletal or stri- 

 ated muscle fibers. Observe that nuclei lie at the periphery of the muscle fibers. (B) 

 Smooth muscle fibers. Upper part of figure shows fibers cut transversely, while lower 

 part represents a longitudinal view of separate fibers. (C) Cardiac muscle. Observe that 

 the fibers appear to anastomose; intercalated discs shown as dark, transverse bands. (D) 

 Connective tissue contributions to skeletal muscle tissue. (E) Diagram of muscle stria- 

 tions. (After Heidenhain.) 



