DYNAMICS OF HISTOGENESIS 353 



The facts of direct observation prove that the formation of muscle 

 tissue is a function of its position. Muscle tissue is formed in situ 

 and is dependent upon an optimum tension, elicited by a dominant 

 zone of accelerated growth, forcing by traction a connected zone 

 retarded in growth. The direction of the resultant muscular fascic- 

 uli serves as a criterion of the direction of the dominant, accelerated 

 force which exerted the tension of differential growth. 



In view of the evidence supporting the conclusion that muscles 

 arise through traction exerted upon the mesenchyme by a force 

 extrinsic to the zone of myogenesis, it is as logical to claim that 

 they self-elongate as it is to assert that a rubber band can stretch 

 itself or that a balloon can self-dilate. In the latter cases extrinsic 

 forces are implied. As regards muscle origin, these forces are 

 elicited by extrinsic zones of accelerated growth, inevitably draw- 

 ing by traction retarded zones of growth, this being due to their 

 relative positions. 



Facts pre^'iously presented b}^ the writer prove that the develop- 

 ing descending colon of the pig embryo possesses two zones of 

 differential growth which by their interaction mutually influence 

 each other during the formation period. The inner epithelial tube 

 is the dominant, most active region of growth. It presents 

 numerous mitotic figures which pursue a path cephalad, primarily, 

 in the manner of a left-handed helix. The outer mesenchymal 

 zone is less active in growth and early in development is composed 

 of a uniform mass of undifferentiated cells. 



The inner, rapidly growing, epithelial tube practically revolves, 

 due to the rapid spiral growth of its cells. With subsequent growth 

 an apparent ring (this is qualified ''apparant," for in reality 

 a close spiral is formed) of smooth muscle myoblasts appears 

 gradually near the periphery of the vortex. 



This position taken by the inner, close spiral, smooth muscle 

 coat at some distance from the epithelial tube is dependent upon 

 an optimum tension. • The attitude heretofore taken by the embry- 

 ologists eliminated the search for the underlying cause of the first 

 formed, inner muscle coat. The theory of self-differentiation 

 excluded interpretation as regards myogenesis. A certain mesen- 

 chymal cell, regardless of position, was considered as destined to 

 become a myoblast. 



