356 EBEN J. CAREY 



quence of which a tensional elongating or stretching action is bound 

 to be exerted upon the surrounding and less actively growing con- 

 tinuous syncytial mesenchyme. It is desired, therefore, to empha- 

 size the following facts: 



First, that there is a dominant energy manifested by rapid skele- 

 tal growth. 



Second, that this energetic zone exerts a tensional or stretching 

 action upon the surrounding mesenchyme, influencing the first 

 steps of myogenesis. 



Third, that the first differentiated muscles react upon the primor- 

 dial blastemal skeleton resulting in a definite series of changes. 

 These are seen in the formation of the condensed cartilaginous 

 skeleton and later, as the muscles become more developed and 

 vigorous, in physiological function in the formation of the osseous 

 skeleton. 



This action and reaction of forming parts results in the condi- 

 tion that at any period of development the degree of differentiation 

 of the musculature and skeleton represents an equilibrium estab- 

 lished between opposing myogenic and skeletal forces. Mechani- 

 cally, therefore, skeletal and the related muscular tissues are inter- 

 dependent, one relying upon the other for its initial and continued 

 differentiation. The interaction of the growing parts in the embryo 

 is considered in an exposition based on experimental teratological 

 evidence by Stockard ('21) . 



Reciprocal elongation of muscles 



The contracted state of a muscle as well as the relaxed arises 

 from a power inherent in itself. The elongation or stretching 

 superimposed on a muscle in tonic contraction depends on some 

 extrinsic power. Simple relaxation of a contracted muscle is not 

 sufficient to enable it to produce another requisite effect. It is 

 necessary that there should be an elongator equal to the quantity 

 of contraction intended to be produced. No muscle has the power 

 of adequately extending or stretching itself; therefore, there must 

 be an elongator. The elongators are usually muscular, but elastic 

 tissue may serve this function as well as fluids in musculo-tubular 

 organs, like the bladder. 



