The Histogenesis of Smooth Muscle in the Alimentary Canal etc. 237 



that whereas in the early embryo the formation of muscle is from 

 primitive mesenchyme, from the 15 mm pig on it is from embryonal 

 coimective tissue. 



10. In the region of muscle formation some of the embryonal con- 

 nective tissue cells do not elongate or form myofibrillae but persist as 

 the interstitial connective tissue cells. The connective tissue, therefore, 

 does not invade the muscle from without but arises in situ. The con- 

 nective tissue nuclei multiply by mitosis. 



11. Although the collagenous fibrillae in development are arranged 

 in a loose reticulum, in the adult this becomes more compact and may 

 e\'en be crowded into more or less continuous, intercellular membranes. 

 These membranes, however, are always fenestrated and through the 

 openings protoplasmic bridges pass from muscle cell to muscle cell. 



12. Many of the collagenous fibers of the adult still retain their 

 primitive, intimate relations with the smooth muscle. They often run 

 from the protoplasm of one muscle cell into neighboring cells, thus 

 binding the tissue firmly together. 



13. Elastic fibers do not appear in the interstitial connective tissue 

 until the embryo reaches a length of 10 mm. They also arise from 

 the syncytium either in the outer protoplasm of the muscle cells or in 

 that of the interstitial connective tissue cells. In cross section of smooth 

 muscle, even in the adult, fibers lying on the surface of the muscle 

 cells are hard to differentiate from peripheral coarse myofibrillae. 



14. In all essential respects, the process of histogenesis of smooth 

 muscle in the respiratory tract is the same as that in the alimen- 

 tary canal. 



