82 DYNAMICS OF HISTOGENESIS. II 



4. In embryos 9.5 to 14 mm. in length the esophageal epitheHal 

 tube grows relatively more rapidly in width than in length. During 

 this period the myoblasts which form the inner, close spiral, muscle 

 coat of the esophagus are becoming rapidly differentiated in the 

 outer condensed margin of the mesenchymal maelstrom. 



5. The nuclei, first spherical then oval, and finally rod shaped with 

 rounded ends, are drawn out in the direction of the circumference 

 of the mesenchymal rim which is directed tangentially. 



6. The cytoplasm is also drawn out in the direction of the mesen- 

 chymal rim of the vortex. The elongated rows of isolated granules 

 appear which subsequently, by confluence, form the myofibrillae. 

 These cytoplasmic derivatives are elongated in the direction of the 

 circumference of the vortex. 



7. Between the epitheHal tube and the myoblastic rim at the peri- 

 phery of the mesenchymal whorl is found the embryonic connective 

 tissue. From this direct observation the conclusion is made that an 

 optimum tensional stress stimulus is necessary to elicit the formation 

 of muscular tissue at the circumference of the mesenchymal vortex. 

 Consequently, the formation of a specific derivative from a pluripo- 

 tent mesenchymal cell is due to the fortuitous circumstance of position. 



8. In embryos from 14 to 24 mm. in length, the esophagus grows 

 relatively more rapidly in length than in width. This elongation is 

 due to two factors; first, the descent of the stomach, and, second, the 

 resistance to diametrical growth presented by the inner close spiral 

 musculature. The epithelial tube, still the dominant zone of mitotic 

 activity, pursues the lines of least resistance, and consequently growth 

 in length takes place. This is due to the shifting of the planes of 

 cell division on account of the compression of the inner, close spiral, 

 muscle coat. 



9. The undifferentiated mesenchyme peripherad to the inner, close 

 spiral musculature is elongated and the histogenetic changes in mus- 

 cular formation are gradually taking place between 14 and 24 mm. 

 A very attenuated, outer, elongated, spiral, or longitudinal muscle 

 coat is detected in the esophagus of a 24 mm. pig embryo. 



10. The characteristic intestino-colic flexure is a torsional reaction 

 of the mesenchyme. The mesenchymal cells are thrown into a left- 

 handed helicoidal series, corresponding to the activity in the epithelial 



