THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 



181 



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 



FlG. 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 necessary, 

 osteoclasts appear. Their origin is not known with certainty. One view is 

 that they are derived from leucocytes, another is that they are derived from the 

 endothelium of blood vessels. Their relation to the myeloplaxes (giant cells) in 

 adult marrow is also a matter of doubt, though it is possible that the two forms 

 are identical. Leucocytes appear in the marrow at an early stage, but whether 

 they arise in situ or are brought in primarily by the blood vessels is not known. 



