368 



DYNAMICS OF HISTOGENESIS. 



in shape through spherical m, elUpsoidal m', and spindle m" cellular 

 phases in Fig. 15. In addition to the homogeneous strain to which 

 the cells m, m' , and m" are subjected there is a definite pressure ex- 

 erted by the epithelial cells lining the expanding lumen. 



Fig. 14. 



Fig. 14. Transverse section of intestine; e, epithelium; m, mesenchyme (spheri- 

 cal nucleus) ; s, peritoneal epithelium. 



Fig. 15. Transverse section of intestine grown to three times the width repre- 

 sented in Fig. 14. Fig. 15 is represented in broken lines within the lumen. The 

 spiral growth of the epithelium is represented by the broken lines sg. The ten- 

 sion, upon the mesenchyme, by the most rapidly growing epithelium, is shown in 

 the elongated muscle cells m." These cells are homogeneously strained in the 

 centrifugal path c' due to the progressively diverging radii. Cells marked w, m', 

 and m" represent the progressive steps in the strain ellipsoid in the differentiation 

 of a muscle from a mesenchyme cell. The expansUe force of the epithelium is 

 shown by the double arrow a-d; the reacting resistance of the serous membrane by 

 the line d-a. Equilibrium is established in the middle of the mesenchyme and is 

 graphically represented by the double arrows a-h and d-c. This is another factor 

 in the tensional elongation of the middle cells. The smooth muscle ring exerts 

 a centrifugal reaction to the applied centripetal force of the dexiotropic spiral 

 rotation of the epithehal tube. The mitotic figures of the descending colon pri- 

 marily follow the path of a left-handed helix. 



