72 DYNAMICS OF HISTOGENESIS. II 



results in the differentiation of muscle tissue is evident. These dif- 

 ferential elements are elongated or arranged in the same direction 

 around the ring. These structures represent the tensional reaction of 

 the mesenchyme to the force of epithelial tubular growth. 



The apparent annular formation of the mesench3rmal cells at the 

 periphery of the vortex is fairly established in an embryo 15 mm. in 

 length (Fig. 4). At a distance the ring appears discrete but when 

 examined with higher magnifications, there is revealed a gradual 

 transition and blending of the degrees of stretching as the cells are 

 examined from the periphery toward the central epithelial tube. The 

 cells which have been subjected to the optimum degree of stretching for 

 muscle origin at the periphery of the mesenchymal whirlpool react by 

 forming myoblasts. Those more centrally placed have been evidently 

 subjected to the optimum tension for the formation of embryonal, 

 connective tissue. From direct observation it appears that the 

 formation of a specific derivative from a pluripotent, mesenchymal 

 cell is due to the fortuitous circumstance of position. Those cells at 

 the periphery of the mesenchymal maelstrom form muscle, those 

 toward the center form embryonal connective tissue. 



The esophagus of an 18 mm, embryo (Fig. 5) shows still more 

 clearly the inner, close spiral musculature, forming an apparent ring. 

 Internal to the ring are found ganglionic clusters of the submucosa or 

 Meissner's plexus; externally are found similar clusters and ramifica- 

 tions of the vagus nerve forming the myenteric or Auerbach's plexus. 

 These are first detected in embryos 12 to 14 mm. in length. 



The submucous, embryonal connective tissue interposed between 

 the musculosa and epithelial tube contains numerous capillaries which 

 by injection methods are demonstrated to pursue a spiral course. 

 These capillaries form from discrete vesicles by confluences at the 15 

 mm. stage. With the development of the inner, close spiral muscu- 

 lature the connective tissue is still more definitely found to be 

 arranged in spirals. 



In the 24 mm. pig embryo (Fig. 6) the external elongated spiral or 

 longitudinal muscle coat is becoming faintly indicated. This is more 

 clearly seen in longitudinal than in cross-sections. This formation is 

 taking place concomitant with the relative and absolute increase of 

 the ratio of the length over that of the diameter of the esophagus. 



