PLATE 1 



EXPLANATION OF FIGURES 



1 Middorsoventral, longitudinal section of hind limb bud of a pig embryo 

 10 mm. in length. X 40. 



2 Middorsoventral, longitudinal section of hind limb bud of a pig embryo 

 12 mm. in length. X 40. 



3 Middorsoventral, longitudinal section of hind limb bud of a pig embryo 

 14 mm. in length. X 40. 



4 Middorsoventral, longitudinal section of hind limb bud of a pig embryo 

 16 mm. in length. X 40. 



5 Middorsoventral, longitudinal section of the thigh of a pig embryo 20 mm. 

 in length. X 40. 



6 Middorsoventral, longitudinal section of the thigh of a pig embryo 25 mm. 

 in length. X 40. 



7 Middorsoventral, longitudinal section of the thigh of a pig embryo 29 mm. 

 in length. X 40. 



ABBREVIATIONS 



d.p.m., dorsal pre-muscle mass 



v.p.m., ventral pre-muscle mass 



a, acetabulum 



il., ilium 



is., ischium 



s.n., sciatic nerve 



/., femur 



.n., femoral nerve 



1, rectus femoris muscle 



2, vastus intermedius muscle 



5, tensile perichondrial strain fibrosis 

 (periosteum) 



4-, Osteogenetic tissue 



6, tensile osseous trabecula 



6, degenerating cartilage cells arranged 

 along tensile and compressive stress 

 lines 



7, proliferating cartilage cells in ad- 

 vance of degenerating zone. 



8, abductor magnus muscle 



9, pyriformis muscle 



10, semimembranous muscle 



11, compressive osseous trabecula 



12, compressive perichondrial strain 

 fibrosis (periosteum) 



dorsal angle, angle formed by a line 

 through the dorsal aspect of the 

 longitudinal axis of the limb with a 

 dorsoventral line through center of 

 hip-joint 



pre-axial angle, angle formed by pre- 

 axial aspect of limb with lateral 

 body wall. By reference to the va- 

 rious changes in these angles the ro- 

 tation of the hind limb is exemplified 

 on a plane surface. Since the aspect 

 of the limb cannot change in these 

 figures, the body axis is represented 

 as changing. 



The attention of the observer is specifically directed to the following facts: 



1. The volume of the central condensed blastemal skeleton (fig. 2) occupies 

 relatively more space of the thigh than the femur in figures 4, 5, 6, and 7. It is 

 immediately evident that as development advances the relative volume of the 

 femur to thigh decreases. At the same time, the density increases as exempli- 

 fied by the progressive deposition of bone (figs. 6 and 7, nos. 5 and 11). The bone 

 on the convex side is drawn out in tension, that on the concave side is compressed. 

 The first deposition of bone, therefore, follows mechanical laws. 



2. The width of the femoral segment is relatively greater in figure 2 than that 

 in figures 4, 5, 6, and 7. The femur grows relatively more rapidly in length than 

 in width in embryos 14 to 25 mm. in length. Note that during the accelerated 



{Continued on p. 112) 



110 



