The Histogenesis of Smooth Muscle in the Alimentary Canal etc. 241 



Fig. 3. Portion of a cross section throiigh the lower oesophagus of a 13 mm pig. 

 Shows the marked elongation of mesenchymal cells, especially of the 

 nuclei, and formation of coarse myofibrillae by condensation and end to 

 end union of granular spindles, gs = granular spindle formed by massing 

 of coarse protoplasmic granules; lis = granules fused into a homogeneous 

 spindle ; vf= homogeneous spindles uniting end to end to form a varicose 

 myofibrilla; fc = coarse myofibrillae; ??« = mesenchymal cells, intimately 

 connected with muscle cells to form a general syncytium ; b = basement 

 membrane; w«;^ = mitosis. Zenker's fluid, Iron-haematoxylin and Congo- 

 red. X 650. 



Plate VIII. 



Fig. 4. Cross section through a portion of the lower oesophagus of a 10 mm 

 embryo. The elongation of some of the mesenchyme cells, with the 

 increase in cytoplasm around the nuclei, shows clearly. In these cells, 

 a faint longitudinal striation is visible in the granular protoplasm. At 

 gs coarser, more deeply staining granules appear, uniting into a granular 

 spindle; e = endoderm; b = basement membrane; m ^mesenchymal 

 syncytium; mu = muscle cell of circular layer; nu = muscle nucleus; 

 pb = protoplasmic bridge between muscle cell and mesenchymal cell; 

 ga = sympathetic ganglion cells (Auerbach's). Zenker's fluid. Iron-haema- 

 toxylin and eosin. x 1425. 



Fig. 5. Cross section through a portion of the lower oesophagus of a 15 mm 

 embryo, showing the rapid formation of the circular muscle layer by 

 elongation and differentiation of the mesenchyme, br = brush of granular 

 myofibrillae, uniting two granular spindles (gs), together. Granular 

 spindles are stained red where they are loosely arranged, black where 

 more condensed, ecs = mesenchymal (embryonal connective tissue) syn- 

 cytium; 7nt = mitosis in an embryonal connective tissue cell. Zenker's 

 fluid. Iron-haematoxylin and eosin. x 1425. 



Fig. 6. A portion of a longitudinal section through the lower oesophagus of a 

 15 mm pig embryo, showing the circular muscle layer cut in cross 

 section. Here the cut ends of the coarse myofibrillae show as black 

 granules. The protoplasmic syncytium is easily made out. fc = coarse 

 myofibrilla. Zenker's fluid. Iron-haematoxyUn and eosin. x 1425. 



Fig. 7. Cross section showing a portion of the circular muscle layer of the lower 

 oesophagus of a 20 mm embryo. The increase in the size of the varicose 

 myofibrillae {vf) is very marked. They are embedded in the compact, 

 granular, protoplasmic syncytium, gm = ganglion of Meissner's plexus; 

 ga = ganglion of Auerbach's plexus; br = shows, well the end of a 

 varicose myofibrilla breaking up into a brush of finer granular fibrillae; 

 hs = homogeneous spindle; ct = embryonal connective tissue cell. 

 Zenker's fluid. Iron-haematoxylin and eosin. x 1425. 



Fig. 8. Longitudinal section through a portion of the lower oesophagus of a 

 27 mm pig embryo, showing beginning elongations for the muscularis 

 mucosae (mm). The longitudinal muscle layer is well differentiated and 

 the circular layer a little thicker than in the 15 mm embryo. The coarse 

 myofibrillae of the longitudinal muscle are seen in various stages forming 

 Internationale Monatsschrift f. Anat. u. Phys. XXIV. 16 



