68 DYNAMICS OF HISTOGENESIS. II 



Figs. 7 to 14. Cross-sections of the trachea and esophagus of a 9.5 mm. pig 

 embryo. Note particularly that the mitotic figures of the trachea follow the 

 path of a right-handed helix, whereas those of the esophagus follow the path of a 

 left-handed helix. These two tubes, therefore, tend to rotate in opposite direc- 

 tions in development. This is comparable to the cylindrical rollers of a printing 

 press. The mesenchymal vortices converge on the right side, represented by 

 arrows, and diverge on the left. This is more evident in older specimens (see 

 Fig. 6). (1) trachea; (2) esophagus; (3) proliferation bud of the trachea. These 

 epithelial cells appear to be thrown off by an eruption and lost in the mesenchyme. 

 At this location the basement membrane of the epithelial tube is absent. The 

 rapid rate of growth of the tracheal epithelial tube proba])ly causes this eruption 

 by reacting centrifugal force. 



