342 EBEN J. CAREY 



view, these muscle types represent differences in the amount of 

 work that has been done upon them by the differential growing 

 parts of the embryo during the active period of growth (Carey, '21) . 

 'The essential difference, then, physiologically between the 

 various muscles is their capacity for work, which in turn is 

 dependent upon the amount of work that has been expended in 

 their production. The reason for the different degrees of energy 

 possessed by the types of muscles is purely an embryological 

 biomechanical problem and corresponds to the differential 

 amount of tensile work that has been expended in their formation 

 by a dominant energetic zone extrinsic to the region of myo- 

 genesis. 



This idea may be exemplified by comparing the heart and the 

 bladder. The rotating blood stream winds up the embryonic 

 cardiac mesenchyme into a spiral musculature for action by the 

 tensional interaction of differential growth, like the key that 

 winds up the spiral springs of an eight-day clock for movement. 

 The tensional stresses exerted by the helicoidal circulation of 

 the blood stream upon the cardiac mesenchyme is the dynamic 

 stimulus for heart-muscle development. The blood gradually 

 collects in the living chick embryo in the region occupied by 

 undifferentiated mesenchyme, which ultimately becomes modi- 

 fied into cross-striated muscle. This modification or transfor- 

 mation is a gradual one. At the thirtieth hour of incubation the 

 primitive mesenchyme begins to rhythmically pulsate, at first 

 slowly, then more and more rapidly as growth continues and as 

 the vascular channels become hollowed out and continuous. 

 This is concomitant with a constantly greater increase in the 

 volume of the blood. Huntington ('11), McClure ('15),Schulte 

 ('14), and others have proved that the vascular channels are 

 formed by the confluence of isolated vesicles due to blood pres- 

 sure. The rate of the heart beat increases from 10 to 15 at the 

 thirtieth hour to 150 to 190 per minute at the ninetieth hour. 

 From the fortieth until the one hundred twenty-fifth hour the 

 heart is composed of cells not unlike smooth muscle. Byt at 

 about the one hundred twenth-fifth hour cross-striations appear. 

 Why? 



