The Histogenesis of Smooth Muscle in the Alimentai-y Canal etc. 221 



gation of scattered embryonal connective tissue nuclei in the sub- 

 epithelial tissue, in a longitudinal direction (Plate VIII, Fig. 8, mm). 

 This elongation does not occur in many nuclei before the 40 mm 

 embryo. Scattered elongated nuclei are occasionally seen running 

 circularly (Plate IX, Fig. 10) and there may be, even in the adult, a 

 few circular fibers, but these are exceptional. In the 76 mm pig, the 

 muscularis mucosae is well developed as shown in Plate IX, Fig. 12, 

 mm, which is drawn from a cross section of the lower oesophagus. 

 In the stomach and intestine both layers of the muscularis mucosae 

 develop at about the same time and appear shortly after that of the 

 oesophagus begins to form. 



As the circular layer of smooth muscle forms, a part of the 

 sympathetic ganglion cells already described are cut off from the rest 

 and come to lie in the mesenchyme between the endoderm and the 

 muscularis (Plate VIII, Figs. 7, 8, gm). These, of course, with their 

 cell processes, form the plexus of Meissner. In the development of the 

 longitudinal layer, the remaining ganglion cells are inclosed between 

 the two muscle layers and form the ganglia of Auerbach's plexus 

 (Plate Vni, Fig. 8, go). In many places bundles of nerve fibers can 

 be seen connecting the ganglia. 



3. Rate and Character of Oroivth of Smooth Muscle. 



As the elongation of mesenchymal and connective tissue nuclei 

 continues in the formation of muscle tissue, there is an increase in 

 the amount of protoplasm surrounding each nucleus (Fig. 4, nu). The 

 perinuclear protoplasmic masses also elongate corresponding to the 

 nuclei, so that the cells change from stellate to spindle-shaped. This 

 change in the shape of the muscle cells does not in any degree do 

 away with the protoplasmic processes or bridges which unite the 

 entire mass of cells into a syncytium (Figs. 4, 8, 15, 19 etc.). In 

 fact, the increase in the protoplasm at the ends of the nuclei tends 

 to make the bridges at these places much larger (cf. Figs. 2 and 3). 

 As will be seen when the later development is discussed, the syncytium 

 persists even in the adult. 



The rapidity of growth of smooth muscle in any given region ot 



