DEVELOPMENT OF MUSCLE. 61 



Development of Muscular Tissue. With the exception of the 

 involuntary muscle connected with the sweat-glands and the muscle-fibres of 

 the iris, all of which probably arise from the ectoderm, muscular tissue is a 

 derivative from the middle germ-layer. The voluntary muscles composing 

 the skeletal group, however, arise from the modified mesoderm that forms 

 the periphery of the quadrate areas, or somites, of the early embryo, while 

 the tracts of involuntary muscle and the heart- muscle are developed from 

 the mesenchyma. The details of their histogenesis vary in each case. Since 

 involuntary muscle is the musculature of the viscera, it generally develops 

 within the walls of tubes. Certain of the mesenchymal cells, the myoblasts, 

 undergo proliferation and elongation and assume the characteristics of the 



Mesentery Mesothelium of serous coat 



Differentiating 

 muscle 



Youngf 

 connective tissue 



Epithelium 

 lining gut-tube 



FIG. 78. Section of developing intestinal wall, showing differentiation of involuntary muscle from 



splanchnic mesoderm. X 180. 



fusiform fibre-cell. Their primary loose arrangement gives place to com- 

 pactness with reduction of the intervening connective tissue. 



The cardiac muscle originates from the nucleated protoplasmic retic- 

 ulum, the syncytium, formed by the mesenchyma of the early heart-tube. 

 Contractile threads, the myo-fibriles, make their appearance within the pro- 

 toplasmic trabeculae and increase in number by longitudinal splitting. The 

 contractile fibrillae are for a time homogeneous, but later undergo differentia- 

 tion in density into light and dark segments, which produce the general 

 transverse striation of light and dark bands. The contractile fibrillae are not 

 uniformly distributed throughout the substance of the trabeculae, but appear 

 in groups that condense into narrow wedges, whose bases lie at the periphery 

 of the fibre, with the thin edges towards the centre. The portion of the 

 cytoplasm which is not converted into fibrillae remains as an undiffer- 

 entiated sarcoplasm, filling the intervals between the groups of contractile 

 threads. After the appearance of the intercalated disks, the trabeculae are 

 subdivided into irregular areas, the so-called muscle-fibres, which by 

 many are regarded as of questionable significance as true structural units. 

 With the progressive increase in the muscular substance, the intertra- 

 becular spaces and the contained embryonal connective tissue rapidly 

 diminish, the former spongy tissue becoming finally condensed into the 

 definitive heart-muscle. 



