66 DYNAMICS OF HISTOGENESIS, II 



Figs. 1 to 6. Development of the esophagus and trachea at corresponding 

 levels through the lower cervical region of pig embryos ranging in length from 9.5 to 

 24 mm. (1) trachea surrounded by a nebular mass of undifferentiated mesenchyme; 

 (2) esophagus, surrounded by a nebular mass of undifferentiated mesenchyme; 

 lumen contains reticular coagulum; (3) left vagus nerve; (4) right vagus nerve; 

 (5) inner close spiral muscle; (6) outer elongated spiral muscle; (7) ganglionic 

 cells of myenteric plexus (Auerbach's plexus) ; (8) ganglionic cells of submucous 

 plexus (Meissner's plexus); (9) intercellular vacuoles of esophageal epitheUum; 

 (10) mesenchyme and embryonic cartilage of the ventral aspect of the trachea. 



Fig. 1. Transverse section through the lower cervical region of a 9.5 mm. 

 pig embryo. 



Fig. 2. Transverse section through the lower cervical region of an 11.5 mm. 

 pig embryo. 



Fig. 3. Transverse section through the lower cervical region of a 14 mm. pig 

 embryo. 



Fig. 4. Transverse section through the lower cervical region of a 15 mm. pig 

 embryo. 



Fig. 5. Transverse section through the lower cervical region of an 18 mm. 

 pig embryo. 



Fig. 6. Transverse section through the lower cervical region of a 24 mm. pig 

 embryo. 



The more rapid growth of the tracheal epithehal tube over that of the esopha- 

 gus is to be especially noted. The mesenchyme around the trachea also grows 

 at a more rapid rate than that around the esophagus. Consequently the less 

 rapidly growing esophageal mesenchyme is thrown into a veritable vortex. This 

 vortex represents a centrifugal reaction of the mesenchyme to the centripetal 

 action of the esophageal epithelial tube. This tube is rapidly growing in the 

 manner of a left-handed helix. At the periphery of the mesenchymal whirlpool 

 the optimum tensional stimulus is presented resulting in the histogenesis of the 

 inner, close spiral, smooth muscle coat. The outer muscle coat is faintly detected 

 in the esophagus of a 24 mm. pig embryo. 



The mesenchyme around the trachea, due to its rapid growth is compressed. 

 This compression results in the formation of cartilage tissue. The embryonic 

 cartilage is becoming evident in the trachea of a 24 mm. pig embryo (Fig. 6). 



The reacting mesenchyme tends to converge between the trachea and the 

 esophagus on the right and diverge on the left. The right vagus nerve appears 

 to be drawn between the two tubes by this action. 



