The Histogenesis of Smooth Muscle in the Alimentary Canal etc. 225 



In the longitudinal layer and muscularis mucosae of the oeso- 

 phagus, in all the muscular layers of the stomach and intestine, after 

 the embryo has reached a length of 30 mm, and in the circular layer 

 of the oesophagus from the 45 mm embryo on, when the second period 

 of new formation begins there, the development of the myofibriliae in 

 the embryonal connective tissue seems to follow a different course 

 from that observed in the younger embryos. Here, however, the 

 formation is more difficult to follow accurately because of the large 

 number of collagenous fibers present. Now, the fine myofibriliae 

 appear to arise directly as such (Figs. 12 and 13) without passing 

 through a granular stage. It is hard to determine absolutely whether 

 this is by a union of the protoplasmic granules in rows and subsequent 

 fusion of these granules; but in most places the fine fibrillae appear 

 homogeneous from the first. The coarse myofibriliae arising in this 

 later formed muscle tissue may develop by a simple growth in caliber 

 of some of the finer myofibriliae or by a union or fusion of the finer 

 fibrillae into compact bundles. 



From the 80 mm embryo on, when the new formation of smooth 

 muscle from embryonal connective tissue is diminished and growth 

 takes place chiefly by mitosis in the smooth muscle already formed, 

 the myofibriliae apparently cease to develop as new structures and 

 probably arise entirely by longitudinal splitting of the myofibriliae 

 already present. There is no conclusive evidence on this point, how- 

 ever. 



The coarse myofibriliae in the later foetal stages differ in some 

 respects from those seen in the early embryo. They are more even 

 in caliber, presenting no spindles or varicosities. In longitudinal 

 sections, when stained favorably, they often can be shown to fray out 

 at the ends and, in places, at the sides, into fine homogeneous fibrillae 

 (Figs. 15 and 17, hr) exactly like the fine myofibriliae lying loose in 

 the protoplasm. In cross sections they appear as shown in Figs. 16 

 and 18 (/b). The deeply staining central mass is the most condensed 

 portion of the fiber, the granular area surrounding this and staining 

 with the protoplasmic stain is made up chiefly of flne fibrillae. It 

 seems probable that these coarse myofibriliae now represent merely 



Iriternationale Monatsschrift f. Anat u. Phys. XXIV. 15 



