366 



DYNAMICS OF HISTOGENESIS. 



Jackson and in that of Keibel and Elze. The former investigator 

 studied the developmental topography of the esophagus, the two lat- 

 ter the histogenesis of the esophagus. Jackson states that the de- 

 scent of the stomach is accompanied by a great elongation of the eso- 

 phagus. In a 9.4 mm. specimen, the esophagus measures 1.8 mm. 

 At this proportion it should measure 4.3 mm. in a 22.8 mm. embryo 

 but its actual length is found to be 8 mm. A year previous to this, 

 Keibel and Elze reported that the esophagus in 12.5 mm. embryos 

 shows a circular but no longitudinal muscle layer. In 17 mm. em- 

 bryos, they find a circular layer with the longitudinal layer faintly 



Ji ml'^i t 



Fig. 12. 



J- ?hli 



Fig. 13. 



Fig. 12. Longitudinal section of intestine; s, peritoneal epithelium; ml, mesen- 

 chyme; m, circular muscle; e, epithelium. 



Fig. 13. Longitudinal section of intestine schematizing the elongation of the 

 intestine represented in Fig. 12. Due to the resistance of the inner smooth muscle 

 layer m, the intestinal epithelium grows in the longitudinal path of least resistance. 

 This results in the elongation of the outer mesenchymal cells ml (Fig. 12) into 

 the elliptical or spindle cells ml (Fig. 13). 



indicated. The histogenesis of the outer longitudinal layer of the 

 esophagus as studied by Keibel and Elze coincides in time with the 

 rapid elongation of the esophagus, due to the descent of the stomach, 

 as recorded by Jackson. 



Interpretation oj Results. 



The result of the action of a force on an elastic body is the produc- 

 tion of a strain. If mechanical forces are at work on organic matter, 

 they tend to produce similar results to these acting upon inert matter. 

 Too frequently the term self-differentiation is applied to alteration of 



