DYNAMICS OF HISTOGENESIS 355 



connective tissues are resultants of certain degrees of stresses. 

 Muscular tissues, on the other hand, are responses to still different 

 types of stresses. The submucosa interposed between the epi- 

 thelial 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 for muscle formation. 



It was observed by von UexkuU that in the nerve net of inverte- 

 brates the excitation flows into a stretched muscle. Therefore, 

 extension, stretching, or elongation of a muscle cell precedes the 

 desired effective contraction, as was inferred long ago by Hunter 

 from observations on mammaUan muscular action. It was also 

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

 supermaximal tensional stimulus to elicit the response of the con- 

 tractile tissue of the stomach in its normal 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. 



The writer has realized (J. Gen. Physiology, '20, vol. 3, p. 63) 

 that the above direct embryological observations would be sub- 

 stantiated by experimentation, as is seen in the following quotation: 

 "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 requisite circumstances, it is 

 transformed into a muscle cell. To this end the writer is directing 

 his attention It must not be forgotton, however, that valuable 

 suggestions pointing to a tensional stimulus as a factor in myogenesis 

 are derived from a study of the origin of this tissue in a closely 

 graded and advancing series of embryos. In the latter case direct 

 observation reveals what is actually going on in nature's own 

 laboratory." 



The above consideration of tensional interaction of differential 

 growth applies to the intestine. The same factor is revealed at 

 work in the differentiation of the skeletal muscles. The primordial 

 blastemal skeleton is undergoing the most rapid growth, as a conse- 



