The Histogenesis of Smooth Muscle in the Alimentary Canal etc. 243 



including a few coarser fibrils; fc = coarse myofibrilla; ff^= fine myo- 

 fibrillae. Zenker's fluid. Mallory's anilin-blue connective tissue stain. 

 X 1425. 



Fig. 14. A portion of the circular muscle layer of the lower oesophagus of a 

 15 cm pig, showing the persistence of the primitive syncytium and the 

 multiplication of the muscle elements by mitosis (m?). Zenker's fluid. 

 Iron-haematoxylin and eosin. x 1425. 



Plate X. 

 Fig. 15. A portion of the circular muscle layer from the lower oesophagus of a 

 10 cm pig embryo, showing the persistence of the cyncytium and the 

 appearance of elastic fibers {el), mt = mitosis in an embryonal connective 

 tissue cell; ct = embryonal connective tissue cells, one of which contains 

 both collagenous fibrillae {cf) and fine myofibrillae (//); pfe = protoplasmic 

 bridges, one uniting two muscle cells and another uniting connective 

 tissue and muscle cells. Zenker's fluid. Iron-haematoxylin and eosin. 

 X 1425. 



Fig. 16. A portion of the circular muscle layer of the lower oesophagus of a 

 15 cm embryo, showing the muscle cut in cross section, /c = coarse 

 myofibrillae, condensed in the center, looser at the periphery where they 

 stain red. Some appear to be breaking up into fine myofibrillae which 

 show as peripheral red granules; ct = embryonal connective tissue cell; 

 cf = connective tissue fibers. Zenker's fluid. Iron-haematoxylin and 

 eosin. X 1425. 



Fig. 17. A portion of the circular muscle layer of the lower oesophagus of a 

 15 cm embryo. This shows a longitudinal section of muscle corresponding 

 to that shown in cross section in Fig. 16. Large numbers of coarse 

 myofibrillae (fc) are visible. Each appear homogeneous in the center and 

 gradually become looser towards the periphery, breaking up into fine 

 myofibrillae (//"); ct = interstitial connective tissue cell; br = brushes 

 of fine fibrillae at the ends of coarse myofibrillae. Zenker's fluid. Iron- 

 haematoxylin and eosin. x 1425. 



Fig. 18. A portion of the circular layer of the lower oesophagus of a 15 cm 

 embryo. Corresponds to Fig. 16 but stained differently, cf = collagenous 

 fibers ; fc = coarse myofibrilla ; ff = fine myofibrillae. Zenker's fluid. 

 MaUory's anilin-blue connective tissue stain, x 1425. 



Fig. 19. A portion of the circular muscle layer of the lower oesophagus of a 

 20 cm pig embryo, showing a marked increase in the number of fine 

 fibrillae (ff) and a decrease in coarse fibrillae (fc); el = elastic fiber; 

 ct = embryonal connective tissue cell through the protoplasm of which 

 run both myofibrillae (ff) and collagenous fibrillae (cf) ; pb = protoplasmic 

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



Fig. 20. A portion of the circular muscle layer of the lower oesophagus of a 

 20 cm embryo, showing the elastic fibers in a connective tissue septum, 

 and in the adjacent muscle. The elastic fibers (el) are seen lying on the 

 surface of the muscle cells (mv),. or between them in the interstitial 

 connective tissue (ct). Zenker's fluid. Weigert's elastic tissue stain, x 1425. 



16* 



