MORPHOGENESIS OF MUSCULAR SYSTEM 705 



myofibrils in smooth muscle fibers do not assume anisotropic (dark) and 

 isotropic (light) bands or cross striations. Increase in the number of muscle 

 fibers (cells) appears to occur by the mitotic division of existing fibers and 

 also by the transformation of other mesenchymal cells. 



C. Morphogenesis of the Muscular System 



1. Musculature Associated with the Viscera of the Body 



The musculature associated with the viscera of the body is of the smooth 

 type with the exception of cardiac muscle and anterior part of the esophagus. 

 Smooth and cardiac musculature are under involuntary control. The smooth 

 muscle tissue of the digestive tract is derived from mesenchyme, which arises 

 from the inner or splanchnic layers of the hypomeres, while that of the urinary 

 and genital systems takes its origin from nephrotomic mesoderm and contri- 

 butions from the splanchnic layers of the two hypomeres (fig. 3 11 A, B). The 

 smooth muscle tissue associated with many of the blood vessels of the body 

 arises from mesenchymal sources in the immediate area of the blood vessels. 



The arrangement of muscle tissue in various parts of the digestive tract, 

 blood vessels, and urinary and reproductive ducts is generally in the form of 

 circular and longitudinal layers (fig. 325B). On the other hand, the myo- 

 cardium or muscle tissue of the heart is an association of layers or sheets 

 which tend to be wound in complex spirals. Particularly is this true of the 

 ventricular portion of the heart (fig. 326A). Also, in the stomach, the arrange- 

 ment of the muscle layers is complex, being composed of an outer longitudinal 

 layer, a middle circular layer, and an inner, somewhat spirally arranged, obhque 

 layer (fig. 326B). The general pattern of arrangement of smooth and cardiac 

 muscle tissues shows much similarity throughout the vertebrate group. 



2. Musculature of the Skeleton 



The skeletal musculature is striated and under voluntary control. It is that 

 musculature which moves various parts of the endoskeleton and integumental 

 structures, enabling the animal to adapt itself to surrounding environmental 

 conditions. The development of skeletal musculature will be described under 

 the following headings: 



(a) development of trunk and tail muscles, 



(b) development of muscles of the head-pharyngeal area, 



(c) development of the musculature of the paired appendages, and 



(d) development of the panniculus carnosus in Mammalia. 



a. Development of Trunk and Tail Muscles 



1) Characteristics of Trunk and Tail Muscles in Aquatic and Terrestrial 

 Vertebrates. In endeavoring to understand the development of the trunk and 



