DYNAMICS OF HISTOGENESIS 343 



On the other hand, the slowly excreted urine which collects 

 in the bladder is the hydrodynamic factor causing the optimum 

 tension for the smooth-muscle differentiation in the vesicular 

 mesenchyme. The volume of the excreted urine that collects in 

 the bladder per unit of time never reaches that of the blood 

 that circulates through the heart in a corresponding temporal 

 interval. Therefore, it is immediately evident that a greater 

 amount of work is expended by the rapidly flowing blood stream 

 which whorls the cardiac mesenchyme into striated muscle than 

 by the excreted urine which causes a gradual tension in the vesic- 

 ular mesenchyme leading to smooth-muscle formation as a 

 mesenchymal reaction. 



The essential difference, then, between the pale smooth muscle 

 of the bladder and the red involuntary striated muscle of the 

 heart is finally dependent upon the differential intensity of 

 hydrodynamic tensi.onal stimuli (work) to which the vesicular 

 and cardiac mesenchymal cells are subjected, respectively. If 

 the growing vesicular smooth muscle is subjected to a stimulus 

 comparable to that found in the heart, will cross-striated muscle 

 be differentiated? The answer is emphatically yes! The fol- 

 lowing experimental evidence puts at rest any further thought 

 that in the embryo some intracellular hypothetical structural 

 precursor within so-called myoblasts accounts for muscle origin 

 as stated by the meaningless verbalism self-differentiation, when 

 applied to myogenesis. 



MATERIALS AND METHODS 



A young female shepherd dog four wxeks old was selected for 

 the experiment. A suprapubic vesicular silver drainage tube, 

 designed by the writer, of the type shown in figures 1, 2, 3, and 4, 

 was transfixed in the bladder, March 30, 1921. This was accom- 

 plished with silkwork-gut ligature which was passed through the 

 perforations in the disc of the tube and tied to exterior of the 

 abdominal w^all. The bladder of this pup presented the pale 

 appearance of smooth muscle on inspection at the initial insertion 

 of the drainage tube. Very little resistance to incision with the 

 scalpel w^as presented by the bladder musculature. The vesicular 

 wall measured 0.5 mm. in thickness. The bladder of a control 



