THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 



177 



principally in the part next the shaft. This produces an elongation of the 

 shaft, the two epiphyses being carried farther and farther apart, and conse- 

 quently a lengthening of the bone as a whole. When the bone reaches the 

 required length, the cartilage disk diminishes and finally is wholly replaced by 

 bone, being represented in the adult only by the epiphyseal line. (See Fig. 158.) 

 MARROW. The forerunner of marrow is the osteogenetic tissue in the pri- 

 mary marrow spaces, which in turn is derived from embryonic connective tissue 

 (Fig. 155). During the development of bone, great numbers of osteoblasts are 



FIG. 158. Longitudinal section from head of femur of young dog. Photograph. 

 The head of the femur is shown in the upper part of the figure, the end of the shaft in the lower 

 part. Between the two the lighter line represents the cartilage between the primary center 

 of ossification (shaft) and the secondary center (epiphysis, head), and marks the site of the 

 epiphyseal line. The lighter portion covering the head represents the cartilage bordering 

 the joint cavity. 



constantly being differentiated from the connective tissue cells and many of 

 these ultimately become bone cells. When development ceases, osteoblasts 

 cease to become differentiated. When dissolution of bone becomes neces- 

 sary, osteoclasts appear. Their origin is not known with certainty. One 

 view is that they represent several osteoblasts which have fused to form a 

 single, large multinucleated cell. Their relation to the myeloplaxes (giant 

 cells) in adult marrow is also a matter of doubt, though it is possible that 

 the polykaryocytes are their direct descendants. The myeloUasts, or pro- 

 genitors of the myelocytes, are approximately spherical cells, larger than white 



