DYNAMICS OF HISTOGENESIS 95 



The reason for incorporating the three headings, involving 

 the development of the muscles, joints, and modifications of the 

 segmental skeleton, in this paper is the fact that the dynamic 

 view-point, as regards limb development, necessitates the con- 

 sideration of concomitant changes, in order to present the 

 sequence of phenomena and evaluate cause and effect. The 

 cause of the fundamental idea, first expressed in this paper, 

 having been overlooked by previous observers is the fact that 

 each tissue has been studied intensively as an entity isolated 

 from the organism as a whole. The entire field of embryology 

 must be reworked from the dynamic view-point of interaction, 

 before a clean-cut idea of the physicochemical endowments of 

 the primordial germ cell may be distinguished from that which 

 is the mechanical resultant of the interaction of differential 

 growth forces. 



The purpose of this paper is to present facts of direct obser- 

 vation which prove that the formation of the skeletal muscles 

 of the hind limb of the pig is dependent upon a tension of the 

 somatic mesenchyme elicited by a force extrinsic to the region 

 of myogenesis. The hind limb, like the gut, possesses two zones 

 growing longitudinally but at different rates. There is an inner 

 blastemal skeleton of accelerated growth and an outer somatic 

 mesenchyme of retarded growth syncytially continuous with 

 the skeleton. By the tensional interaction of this differential 

 growth of the limbs, the skeletal muscles arise. 



2. DIRECT OBSERVATIONS ON THE EARLY DEVELOPMENT OF THE 

 HIND LIMB OF THE PIG (SUS SCROFA) 



In embryos 6 to 10 mm. in length (fig. 1) the hind limb is 

 represented by a convex bud covered with ectoderm and filled 

 with a nass of uniform cells manifesting no signs of differentiation. 

 As development advances, in embryos 10 to 14 mm. in length, 

 a rapid condensation of the blastemal skeleton, proximodistally 

 is occurring pari passu with limb extension. The nuclei of the 

 syncytial skeletal core undergo rapid mitotic division. The 

 first increase in size of the blastema is relatively more rapid in 

 width than in length (fig. 2 and table 2). 



