276 MICROSCOPIC ANATOMY OF THE 



Development of Muscles. 



In the early study of the growth of muscles it was claimed 

 by some investigators (Schwann, Valentine) that muscle fibres 

 are built up by the fusion of many indifferent cells. Rernak 

 claimed that the muscle fibre is derived from a single cell, a view 

 which since has gained general acceptance. The way in which 

 such fibres are joined together in the embryo to form definite 

 muscles is not satisfactorily understood. Certain facts, how- 

 ever, have been obtained as to the growth of embryonic muscles. 

 The development of the human sartorius muscle has been 

 worked over in recent years (MacCallum). At an early stage 

 the cells making up the muscle are small and spindle-shaped, 

 and are scattered in loose bundles. At first there are no fibril 

 bundles, and the nucleus is placed centrally. Subsequently 

 the fibril bundles appear around the periphery of the cell. 

 The cells become more numerous and increase in size until 

 the human embryo is between 130 mm. and 170 mm. in 

 length from vertex to breech. At this stage the bundles of 

 cells become more compact and the cells themselves are filled 

 with fibril bundles as in the adult. The fibres now grow in 

 length and thickness, but no longer increase in number. In 

 embryos smaller than 170 mm. in length there is a progressive 

 increase in the number of fibres found in a cross-section. After 

 this, however, the number remains approximately constant. 

 In other words, the fibres of the human sartorius do not in- 

 crease in number after about the first half of embryonic life. 

 After this period the increase in size of the muscle is due to 

 growth of the individual fibres, and not to their multiplica- 

 tion. 



Marpargo has observed that in the white rat the muscle 

 cells continue to multiply for a short time after birth. Ac- 

 cording to Meek, hyperplasia of the muscle cells ceases at 

 birth, and after this there are a reduction in the number of 

 fibres and a hypertrophy of those remaining. 



A vascular connective tissue separates the muscle bundles 

 to form primary and secondary groups, which, according to 

 Bardeen, are to be considered as units. 



