The Histogenesis of Smooth Muscle in the Alimentary Canal etc. 233 



with protoplasmic stains than do the fine myofibrillae during develop- 

 ment, and iron-haematoxylin is likewise retained by them more readily. 

 In cross section, they appear as fine dots (Plate XI, Figs. 25 and 26, ff), 

 closely packed together. 



In the adult smooth muscle, both of the alimentary and of the 

 respiratory tracts, coarse fibrillae may or may not be present (Plate XI, 

 Figs. 21 to 26 and 29). When present they often appear as bundles 

 of fine fibrillae. When these bundles are loosely arranged, the fine 

 fibrillae of which they are composed may be demonstrated, especially 

 at the ends, where they break up into loose brushes (Fig. 23, òr). In 

 cross section the coarse myofibrillae often appear as bundles of fine 

 fibrillae (Figs. 25 and 26). Some of the coarse myofibrillae, however, 

 stain as solid fibers (Fig. 23, /b; also in Figs. 26 and 29). 



The coarse myofibrillae appear, as a rule, to be less numerous in 

 the adult than in the later foetal stages, though occasionally vrery 

 numerous (Plate XI, Fig. 29). In position, they are seen in cross sections 

 of adult muscle lying partly scattered through the muscle cell, and partly 

 near the periphery or at the surface (Figs 25, 26 and 29). 



The coarse fibrillae, especially those lying close to the periphery 

 of the muscle cell, have to be differentiated from the elastic fibers, 

 which lie in the connective tissue immediately surrounding the muscle 

 protoplasm (Figs. 26, 28, el). This is especially true in material stained 

 with iron-haematoxylin, where both coarse myofibrillae and elastic 

 fibers stain intensely black. For instance, in the cell marked {mu) in 

 Fig. 25, it is impossible to tell whether some of the peripheral fibers 

 are elastic fibers or coarse myofibrillae. In longitudinal sections it is 

 usually comparatively easy to tell them apart, for the elastic fibers 

 have more regular outlines and are usually more wavy (Figs. 21, 22, 

 23, el) than the coarse myofibrillae (Fig. 23, fc). Moreover, the elastic 

 fibers lie either out in the interstitial connective tissue or else right 

 at the margin of the muscle protoplasm, while the coarse myofibrillae 

 may be scattered throughout the cell protoplasm. It is possible that 

 at least a portion of the „Grenzfibrillen" described by Heidenhain 

 (1898) as lying at the periphery of the muscle fiber are elastic fibers, 

 though I have not studied the amphibian muscle. 



