98 



STUDIES IN ADVANCED PHYSIOLOGY. 



twenty-first year, and at the upper end about the twenty- 

 fourth year. The individual vertebrae which fuse together 



Fig. 44. PART OF A LONGITUDINAL SECTION OF THE DEVELOPING FEMUR OF THE RAB- 

 BIT. DRAWN UNDER A MAGNIFYING POWER OF 350 DIAMETERS. (From Klein and 

 Noble Smith.) 



a, rows of flattened cartilage-cells; b, greatly enlarged cartilage-cells close to the ad- 

 vancing bone, the matrix between is partly calcined; c, d, already formed bone, the os- 

 seous trabeculae being covered with osteoblasts (<?), except here and there, where a giant- 

 cell or osteoclast (/), is seen, eroding parts of the trabeculae; g, h, cartilage-cells which 

 have become shrunken and irregular in shape. From the middle of the figure downwards 

 the dark trabeculae, which are formed of calcined cartilage matrix, are becoming covered 

 with secondary osseovis substance deposited by the osteoblasts. The vascular loops at 

 the extreme limit of the bone are well shown, as well as the abrupt disappcarmu-c of the 

 cartilage-cells. 



to form the sacrum, and which are ossified from five differ- 

 ent centers, unite as late as the twenty-fifth year, while the 



