EBEN J. CAREY 369 



This force of expansion is represented by the arrow a-d. A re- 

 sistance /, due to the peritoneal epithelium, is met. This causes a 

 reaction d-a, With progressively increasing growth a zone of equili- 

 brium of expansile and reacting forces is estabHshed in the middle, 

 represented by the double arrows h-a, and d-c. This action and re- 

 action of forces is another factor tending to compress the cells in the 

 middle of the mesenchymal wall of the intestine. The action of the 

 centripetal component of the spiral growth of epitheHum in forming 

 the rings of dense spindle-shaped muscle cells m" is represented by the 

 broken curved arrows. The spiral growth of the epithelial tube in a 

 dexiotropic rotation exerts a centripetal force upon the surrounding 

 mesenchyme. The mesenchyme consequently exerts a simultaneous 

 equal and opposite centrifugal force upon the epithehal tube. This 

 growth is primarily in the form of a left-handed helix from the rectal 

 to the ileocecal valvular regions of the large intestine. In Figs. 14 

 and 15 the right-handed helix is depicted. 



Although by direct observation of serial sections no motion is seen, 

 there is, however, objective evidence of homogeneous and ellipsoidal 

 strains upon the surrounding mesenchyme. The mesenchyme is 

 drawn out into concentric rings, the outermost of which are most 

 viscid, by the spiral growth of the epithelial tube, roughly comparable 

 to the increase of viscosity and concentric annular formation of 

 egg albumin when subjected to an egg-beater. 



CONCLUSION. 



The genesis and maintenance of muscle tissue represents a result- 

 ant or equilibra:tion of converging factors which are active and forma- 

 tive during development. One of these factors is the tensional stresses 

 to which the mesenchyme is subjected by a force extrinsic to the diferen- 

 tiating zone. In subsequent involution or degeneration of muscu- 

 lar tissue during the prenatal or postnatal periods, this equilibrium is 

 upset by altering or destroying the tensional reacting stress. 



Tension is developed when a muscle contracts. Contractility is a 

 fundamental property of protoplasm and, when manifested, tension 

 is developed. In both, the development and specific function of 

 muscle tissue, tensional stresses are inseparably involved. The 



