EBEN J. CAREY 63 



The submucosa interposed between the epithelial tube and the inner 

 smooth muscle coat presents cells which react to a certain minimum 

 of tensional stress. Just peripheral to the submucosa, muscular tissue 

 is differentiated as a response to an optimum tensional stress. 



It was observed by von Uexkull that in the nerve net of inver- 

 tebrates the excitation flows into a stretched muscle. Extension, 

 stretching, or elongation of a muscle cell precedes the desired 

 effective contraction, therefore, as was inferred long ago by Hunter 

 from observations on mammahan muscular action. It was also 

 found by Cannon that there was a subliminal, an optimum, and a 

 supermaximal tensional stimulus to elicit the response of the contractile 

 tissue of the stomach in the normal stomachic movements. Evidently, 

 an analogy is here found for the development of the musculature. 

 There appears to be a subliminal, an optimum, and a supermaximal 

 tension for stimulating the formation of contractile tissue. In normal 

 development as well as in subsequent normal function the tensional 

 stresses appear to be fundamentally involved. 



Tension is due to a definite mechanical action. The formation of 

 muscular tissue is due, therefore, to a definite active process, not a 

 passive one as the term self-dift'erentiation connotes. Loeb like- 

 wise concludes from his experiments on the gastrocnemius muscle of 

 the frog that growth is an active, not a passive, affair as follows: 

 "Activity, therefore, plays the same role in the growth of a muscle 

 that the temperature plays in the growth of the seed." 



The dominant growth of the epithelial tube and the resultant ten- 

 sion or stretching of the surrounding mesenchymal cells is strikingly 

 exemplified throughout the digestive tract in its early stages of de- 

 velopment. The object of the first part of this paper, therefore, is 

 to demonstrate this interaction in the esophagus; furthermore, to 

 interpret certain facts of subsequent torsional development of the ali- 

 mentary tract in the light of the growth of the epithelial tube in the 

 manner of a left-handed helix and the reaction of the mesenchyme to 

 this epithelial growth. The origin of the spiral epithelial growth is 

 also briefly considered. 



To an advocate of the experimental sciences it is undoubtedly 

 necessary that an actual experiment should be made showing that 

 by gradual stretching of a cell, under the requisite circumstances, it 



