DYNAMICS OF HISTOGENESIS 97 



During this period of rapid interstitial growth in length of 

 the femoral blastemal center of the segmental skeleton, resist- 

 ances are met with, first, at the proximal end in the centers of 

 opposed growth for the ilium, ischium, and pubis; next, at the 

 distal end in the centers of opposed growth for the tibia. The 

 less condensed region, between the centers opposed in growth, 

 becomes more and more condensed until a compression of the 

 blastemal cells occurs along the line of juncture of the centers 

 opposed in growth. Certain observers consider the segmenta- 

 tion of the skeleton as taking place at this stage and view the 

 compression line as representing the beginning of the hereditary 

 joints. The appearance of the joints, however, is the mechanical 

 result of the opposed growth of antecedent, contiguous accel- 

 erated growth segments, and not the spontaneous cause of the 

 segmentation. 



With the inception of the outlines of, first, the hip and, next, 

 the knee-joints, the contour of each skeletal segment becomes 

 more defined. During this period the blastemal cells toward 

 the center of the femoral segment become chemically changed by 

 the intercellular formation of a cartilage matrix. This gives each 

 cell the clean-cut appearance of a cell membrane. At the same 

 time the primordial femur is bent, with its convexity toward 

 the quadriceps extensor muscle and the concavity toward the 

 hamstrings. The resistance to interstitial growth in length 

 of the femur, due to the opposed centers at the acetabulum and 

 at the knee, together with muscular contractility now mani- 

 fested in the inception of the normal rotation of the hind limb, 

 causes this characteristic bend of the /emoral rod. 



The attention of the observer is directed especially to the 

 significant fact, that all cellular differentiation and degeneration 

 occur first on the convex aspect at the center of the bent beam 

 (fig. 5, no. 4). The first change from the blastema is the trans- 

 formation into cartilage. Soon a dense perichondrial strain 

 fibrosis is formed, followed by the degeneration of the cartilage 

 cells and a calcification of the cartilage matrix. These retro- 

 gressive changes are due to the constriction of the blood supply, 

 readily proved by injecting the embryos with india ink while 



