VI. EFFECTS OF DEFICIENCY 233 



mediate formation of cartilage.^- What is originally membrane thus becomes 

 cortex of the new bone. 



The second type of bone, which includes all the bones not listed above, is 

 known as cartilage bone. In this type of bone, growth takes place Avith the 

 intermediation first of cartilage formation and then destruction of the 

 cartilage l:)ef()re lione can be formed. In the prenatal formation of these 

 bones there is first an aggregation or condensation of mesodermal cells at 

 the location of the future bone. This aggregation of cells becomes demar- 

 cated from the surrounding m,esoderm and forms a rudimentary model of 

 the future bone. In the area of the futiu'e shaft the peripheral cells become 

 fil)roblasts and form the periosteimi, while at the ends of the future bone 

 the cells form the periochondrium. 



In the region of the shaft, where the membranous periosteum has formed, 

 bone formation proceeds in the manner described for membranous bone. 

 Elongation of the bone takes place at its ends, where the condensation of 

 mesenchymal cells has resulted in the formation of a cartilaginous plate 

 which (hu'ing childhood remains distinct from the shaft, and is called the 

 epiphysis. The flattened cells in this cartilaginous plate are normally ar- 

 ranged in columns. Each cell in the column is separated from its fellow by 

 a thin bridge of matrix, while the adjacent columns are separated by wider, 

 parallel bands of matrix material. The epiphysis is attached to the diaphysis 

 by a number of calcified prongs remaining from those destroyed which form 

 a bridge between epiphysis and diaphysis. 



As one proceeds from the ielati\'ely (juiescent epiphysis toward the zone 

 of ossification, the appearance of the individual cells changes. The rows 

 of cells most distant from the shaft are composed of cells hardly differenti- 

 able from the ordinary cartilage, and they form the zone of resting cartilage. 

 As we proceed in the direction of the shaft or diaphysis toward the second 

 layer, the cells begin to show evidence of degeneration. The mitochondria 

 appear rod-like, and later shrink; vacuoles containing fat and glycogen 

 appear, and the cell nucleus shrinks. This zone, the zone of proliferating 

 cartilage, varies in depth, depending on the rate of bone growth. In the 

 third layer, the zone of preparatory calcification, the process of cell disinte- 

 gration is complete. Here Ave find clumps of degenerated cells, or only 

 lacunae from which the cells have disappeared. At the same time, if there 

 are adequate concentrations of calcium and phosphate in the blood plasma, 

 the matrix around the cells undergoes a notable change. There is a pi'ecipi- 

 tation of calcium salts which results in the matrix walls' becoming calcified. 

 Immediately to the shaft side of this area of i)reparatory calcification, the 

 final stage of actual ossification takes jjlace. This process is due to the 

 activity of the osteoblasts and blood vessels of the hone marrow. The cells 



^- E. A. Park, Personal ccMuniuiiication. 



