144 THE TISSUES. 



At its junction with tendon the muscle-fiber with its sarcolemma 

 is rounded off into a blunt point, the fibrils of the tendon being 

 cemented to the sarcolemma. 



The longitudinal growth of muscle-fibers takes place principally 

 at the distal ends of the fibers, at which point their nuclei are numer- 

 ous. (Fig. 103, at a.) Schaffer (93, II) has recently suggested that 

 there is a formative tissue between the tendon and muscle-substance, 

 from which, on the one hand, muscle-fibers are developed, and, on 

 the other hand, connective-tissue fibrils and cells are formed. 



As recent investigations have shown, the development of muscle 

 continues throughout the life of the individual. Muscular tissue is 

 consequently to be regarded as in a perpetual stage of transition, 

 the destruction and compensatory reproduction of its elements going 

 on hand in hand. Its destruction is ushered in by a process which 

 can be compared to a physiologic contraction. Nodes or thickened 

 rings are formed, and at these points the muscle-substance separates 

 into fragments with or without nuclei (sarcolytes), which are then 

 absorbed, in most cases without phagocytic aid. This loss of sub- 

 stance is replaced by new elements developed from the free sarco- 

 plasm, which is characterized by rapid growth and increase in the 

 number of its nuclei. The result is that new elements are formed 

 which have been called myoblasts. The process by which myo- 

 blasts are changed into the finished muscle-fibers is exemplified in 

 the embryonal type of development of the tissue. 



Development of Voluntary Muscle-fibers. The striated, 

 voluntary muscular tissue, as above stated, develops from the myo- 

 tomes, segmentally arranged differentiated portions of the meso- 

 derm. In the myotomes are developed round or oval cells known 

 as myoblasts, which proliferate by mitotic cell division. According 

 to the observations of certain observers, the myoblasts elongate and 

 become spindle-shaped, while the nuclei proliferate, without an ac- 

 companying division of the cell body, to form the muscle-fibers, 

 which may thus be regarded as polynuclear cells developing from 

 a single cell. Other observers, notably Godlewsky, state that only 

 relatively few of the muscle-fibers develop in this way, the majority 

 being formed by a fusion of myoblasts, forming a syncytium, a 

 muscle-fiber being thus a syncytial structure developed from a vary- 

 ing number of myoblasts. 



The contractile fibrils are differentiated from the protoplasm of 

 the differentiating myoblasts. When first seen, they present a uni- 

 form structure, and only later can a differentiation into isotropic and 

 anisotropic substance be recognized. The discs Q and j appeal- 

 first ; the other parts of the sarcomeres somewhat later. The first 

 formed fibrils divide longitudinally to give rise to new fibrils. Em- 

 bryonic striated muscle tissue, even after striation of the fibers may 

 be observed, forms a very compact tissue with only narrow inter- 

 spaces between the cells. In the further development of this tissue 

 certain of the embryonic muscle-fibers undergo degeneration 



