DYNAMICS OF HISTOGENESIS 361 



3. The essential difference between the pale smooth muscle of the 

 bladder and the red involuntary striated muscle of the heart is dependent 

 upon the differential intensity of hydrodynamic tensional stimuli to 

 which the vesicular and cardiac inesenchymal syncytia, respectively, 

 have been subjected during development. 



4. The evidence herein presented proves definitely that the pale bladder 

 musculature may be transformed into the red, cross-striated type by 

 increasing the tensional stimidus to a degree comparable with that 

 which the cardiac mesenchyme experiences normally. 



5. Muscle tissue is not a self -differentiated product, but is a bio- 

 mechanical resultant of an optimum tension. The variable intensity 

 of the optimum tension determines the muscular type. The growing 

 cells receive and respond to the mechanical tensional stimulus. The 

 stimulus, however, is a function of position. 



6. In considering the origins of the heart beat, the extrinsic 

 hydrodynamic tensional stimulus as well as the irritable reacting 

 body — the heart muscle — is shown to be absolutely necessary as one 

 of the factors accountable for heart rhythm. 



7. The evidence herein presented proves that the structure of 

 striated muscle is determined by the function it performs and the 

 work it does and that cross-striated muscle is not formed in antici- 

 pation to a future function. The conclusion is warranted that 

 function in this case determines structure, and not the reverse. 



In addition to assistance received from his wife and Dr. McGrath, 

 as mentioned above, the author is also indebted for help to Dr. 

 Benj. Schlomovitz and Mr. Leo Massopust, the departmental 

 artist. To Dr. C. R. Bardeen, of the University of Wisconsin, 

 the author's thanks are due, for his most encouraging interest 

 in this problem. 



