114 HISTOLOGY 



The formation of smooth muscle may be studied advantageously in 

 the oesophagus of pig embryos, and its development in this position 

 has been carefully described by Miss McGill (Internat. Monatschr. f. 

 Anat. u. Physiol., 1907, vol. 24, pp. 209-245) A part of a longitudinal 

 section of the oesophagus of a 27-mm. pig is shown in Fig. 103. In such 

 a section the developing longitudinal smooth muscle fibers or myoblasts 

 are cut lengthwise (s.l.) ; and the circular fibers, which form a layer internal 

 to the longitudinal fibers, are cut across (s.c.). The loose mesenchymal 

 network, from which these fibers arise, is continuous with them above 

 and below. A third thin layer of muscle fibers is forming at m.m., and 

 at the top of the figure, the entodermal epithelium which lines the 

 oesophagus has been included, together with the basement membrane 

 beneath it. 



In becoming smooth muscle cells, the mesenchymal cells change from 

 a stellate to a spindle-shaped form and come closer together, but they do 

 not lose their protoplasmic connections with one another. In the outer 

 part of their protoplasm coarse border fibrils or myoglia fibrils are produced, 

 which are similar to the fibroglia fibrils of connective tissue (p. 64). 

 According to Meves, the fibroglia and myoglia are identical. The latter 

 are at the periphery of the muscle cells and pass from one cell to another 

 for long distances. These fibrils may be strikingly demonstrated in the 

 oesophagus of a 24-mm. pig, stained with phospho-tungstic acid haema- 

 toxylin. 



The coarse fibers shown by Miss McGill in both the circular and longi- 

 tudinal muscle layers in Fig. 103 are "often found lying in part near the 

 surface of the cell, resembling the border-fibrils of Heidenhain." She 

 states that they are produced by a coalescence of granules within the 

 protoplasm, forming at first spindle-shaped bodies which later join end 

 to end, making varicose fibers. Subsequently they become smooth. 

 They may split into fine fibrils, and usually they decrease in number as 

 the embryo grows older. "In places they may be entirely absent in the 

 adult tissue; rarely they are abundant." 



In addition to the peripheral myoglia fibrils, the protoplasm of smooth 

 muscle cells contains fine longitudinal fibrils, which have been de- 

 scribed as the active agents in muscular contraction. Thus Miss McGill 

 finds that in the contracted portions of the muscle fibers the myofibrillae 

 show "a distinct increase in caliber." She states that the fine myofibrils 

 do not arise until the pig embryo reaches a length of about 30 mm. They 

 are apparently homogeneous from the beginning, and are distributed uni- 

 formly throughout the protoplasm. Some of them are shown in the muscle 

 layer m.m. in Fig. 103. Ordinarily these fibrils are indistinguishable in the 

 close-grained, deeply staining protoplasm which characterizes the muscle 

 cells. 



