EBEN J. CAREY 367 



form and internal structure of developing cells without searching the 

 immediate environment of the specializing cells or syncytium to as- 

 certain whether or not these changes are attributable to forces out- 

 side of the differentiating zone. This applies particularly to the dif- 

 ferentiation of bone and muscle tissue. If a cell changes in form suc- 

 cessively through the spherical, ellipsoid, and spindle stages it under- 

 goes a strain. A strain is usually due to an external force which elicits 

 internal reacting stresses in the body acted upon. Cytological dif- 

 ferentiation is frequently a manifestation of these internal reacting 

 stresses to forces extrinsic to the differentiating zone. 



A strain is produced in certain regions of the embryo by the expan- 

 sion of a rapidly dividing group of cells against a less active or rela- 

 tively passive group of cells. After their differentiation the relatively 

 passive group of cells in their turn react upon the former. This ac- 

 tion and reaction are objectively evident by a retardation or altera- 

 tion of the rate of growth, or by a change produced in the external 

 form or internal structure of the cells involved. 



In this study, the initial zone of rapid growth is found in the epi- 

 thelial tube. The rapid spiral expansion of the entodermal epithelial 

 tube reacts against the surrounding splanchnic mesenchyme with the 

 result that the less actively growing cells of the peripheral region of 

 the intestinal wall are elongated. Later the elongated, differentiated 

 mesenchjnnal cells cause a retardation of the growth in diameter of 

 the epithelium. Immediately following this retardation of diametri- 

 cal growth the period of rapid growth in length of the intestine takes 

 place. In this development, therefore, the influence of unequal 

 growth zones is definitely shown as furnishing a tensional stimulus for 

 the differentiation of muscle. 



This action is diagrammatically illustrated in Figs. 14 and 15. In 

 Fig. 15 the growth in diameter of the intestine is schematized; the 

 rapid increase in width is shown as due primarily to the increase in 

 the lumen. This growth is due to rapid mitotic activity of the epi- 

 thelium {e, Fig. 14, to e', Fig. 15). In the lumen of Fig. 15, the former 

 is represented in a spiral manner, sg. In this growth the strain upon 

 the surrounding mesenchymal cells m is illustrated. These cells are 

 strained by the external applied forces of the progressively diverging 

 radii. The internal reacting stresses are manifested by the changes 



