THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 145 



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. 120.) 

 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. 117). During the development of bone, great numbers of osteoblasts are 



FIG. 1 20. 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 jomt 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. Marrow is one of the chief centers of blood 

 cell formation in later foetal life, and in the adult is normally probably the 

 only source of erythrocytes. An account of blood cell formation will be 

 found in the section on "Haemopoiesis." The myeloblasts, which are prob- 

 ably identical with or at least closely allied to the primitive blood cells 

 (haemoblasts), by acquiring certain types of granules in the cytoplasm 

 become neutrophilic, acidophilic or basophilic myelocytes. During develop- 

 ment two types of giant-cells (myeloplaxes) appear in the marrow. Accord- 



