DYNAMICS OF HISTOGENESIS 101 



With the progressive increase of femoral density the definitive 

 muscles of the thigh become split from the dorsal and ventral 

 premuscle masses. Those myoblasts favorably located along 

 lines of optimum tension for continued muscle differentiation 

 advance in development; whereas, those not so favorably situ- 

 ated revert to embryonal connective tissue. This split into the 

 separate muscles begins with the first compression line forming 

 the hip-joint. Those mesenchymal cells previously drawn out 

 opposite the location of the future joint by the indefinitely 

 outlined blastemal femur retrogress with the progressive clean- 

 cut development of the joints. This is due to the fact that the 

 lines of optimum tension become more definite with the forma- 

 tion of the joints and with the continued development of the 

 definite contour for the femur. 



The premuscle mass for the quadriceps extensor (fig. 3) is 

 seen to present a cleavage line opposite the dorsal aspect of 

 the developing hip-joint composed of retrogressive myoblasts. 

 With the continued morphogenesis of the femur and hip-joint, 

 the dorsal premuscle mass is seen to split into its derivatives. 

 The two muscle derivatives clearly seen in figures 4, 5, and 6 

 are the vastus intermedins and the rectus femoris. Note that 

 the cleavage separating these muscles in the dorsal premuscle 

 mass begins opposite the hip-joint (fig. 4) and progressively 

 advances toward the patella (figs. 5, 6, 7). 



The progressive elaboration of the hip-joint advances pari 

 passu with the increasing density and the interstitial growth of 

 the femur, together with the developing definitive thigh muscles. 

 The head of the femur advances farther and farther into the 

 acetabulum formed by the ilium, ischium, and pubis. By 

 reference to figures 3, 4, and 5, it is seen that the growing segment 

 forming the femur possesses a greater longitudinal growing force 

 per square millimeter of cross-section than that of the elements 

 forming the acetabulum. By actual measurement at the 20-mm. 

 stage of the embryo, the length of the longitudinally growing 

 femur is 2.40 mm., whereas the acetabulum possesses a depth 

 of 0.195 mm. This is significant, for the force of longitudinal 

 interstitial growth of the femur per square millimeter of cross- 



