Charles E. Bardeen 289 



accurate description of the processes which take place. Of the more 

 recent articles that of Kazzander, 94, deserves special mention. 



Until the embryo reaches a length of about 17 mm. the knee-joint is 

 marked by a dense mass of tissue, Fig, 59. The medullary tissue at the 

 knee, like that at the hip and other joints, is less dense than the surround- 

 ing cortical substance, so that when the cartilages of the femur, tibia and 

 fibula are first differentiated they seem to be connected by a tissue which, 

 in some respects, resembles the prochondrium of which they are com- 

 posed. Fig. 55. But as the cartilages become more definite the apparent 

 continuity disappears. As the musculature becomes differentiated a 

 dense tendon for the quadriceps is formed in front of the knee-joint. 

 This is shown well in Fig. 56. In it the patella becomes differentiated. 



In embryos of about 20 mm. the tissue immediately surrounding the 

 cartilages becomes greatl}^ condensed into a definite perichondrium. The 

 peripheral blastemal tissue at the joints becomes transformed into a 

 capsular ligament strengthened in front by the tendon of the quadriceps. 

 Within the joint most of the tissue begins to show signs of becoming less 

 dense, Fig. 56, but the semi-lunar disks and the crucial ligaments, like 

 the ligaments of the capsule are differentiated directly from tlie blastema. 

 Figs. 61 to 65. A knee-joint cavity first appears in embryos about 

 30 mm. long. 



The shafts of the tibia and fibula are incompletely separated in the 

 blastemal stage. The cartilages which arise in the scleroblastema are, 

 on the other hand, separated by a distinct interval, Fig. 50, and as the 

 blastemal elements give way to cartilage the interosseous space becomes 

 larger. This is shown in Figs. 3, 4, 5, 6, 9, 10, 11, 12 and 59. At first 

 short and thick the shafts gradually become more slender in proportion 

 to their length. The fibula at all times smaller, becomes increasingly 

 more slender in comparison with the tibia. In embryos of 30 mm., 

 Figs. 11 and 12, both bones, and especially the fibula, are relatively thick 

 compared with the adult bones. 



During a period of rapid development, in embryos of 15 to 20 mm., 

 the tibia and fibula, like the femur, may extend so rapidly in length as to 

 become temporarily distorted by resistance at the ends. This is often 

 especially marked in hardened specimens. Holl, 91, Schomburg, 00, 

 and others have called attention to this distortion. 



Ossification begins in the tibia at about the same time that it does in 

 the femur and a little earlier than it does in the fibula. It is usually 

 under way in embryos 25 mm. long. In older embryos it is generally 

 well marked. Figs 11 and 12, Plate V. It begins in both bones knee- 

 wards from the center of the shaft and from here spreads toward the 



