58 THE TISSUES. 



growing bone become osteoblasts and aid in bone-formation ; 

 (b) large multinucleated giant cells, or inyeloplaxes, which are 

 probably osteoclasts and effect bone- absorption ; (c) crytkro- 

 blasts, free nucleated cells, colored by and containing haemo- 

 globin, probably connected with the development of red blood- 

 corpuscles ; and (d) cells containing oxyphile granules, re- 

 sembling the eosinophile leukocytes of normal blood; 

 other cells resembling the myelocytes which appear in the 

 blood in certain abnormal conditions ; and other forms of leuk- 

 ocytes. 



Yellow marrow occurs in the medullary cavities of long 

 bones and larger cancelli of the adult. It is similar to red 

 marrow, except that it contains large quantities of fat-cells, or 

 marrow-cells distended with fat, to which it owes its oiliness 

 and straw color. 



The function of marrow is partly to form bone-substance, 

 partly to form red blood-corpuscles. Genetically the marrow 

 is a continuation or offshoot of the osteogenetic periosteal 

 layer. 



The development of bone takes place by two methods, carti- 

 laginous and membranous. 



Cartilaginous bone-formation (Fig. 26) : Most of the bones, 

 especially the long bones, are in the embryo first formed of 

 hyaline cartilage. The conversion of this cartilage into bone 

 constitutes the cartilaginous method of bone-formation. In 

 these embryonic cartilages bone-formation begins at definite 

 points, the " centres of ossification." The cartilage-cells 

 enlarge and proliferate abundantly, and arrange themselves in 

 longitudinal rows, with septa between the rows composed of 

 the hyaline matrix. The longitudinal spaces inclosed by these 

 septa and filled with cartilage-cells are called the primary 

 areolce. These septa soon become calcified by the deposition 

 in them of lime salts. From the osteogenetic layer of the 

 periosteum loops of bloodvessels then grow into the cartilage, 

 accompanied by connective-tissue cells, and by large multi- 

 nucleated cells called osteoclasts, which have the power of 

 absorbing the cartilage and bone-substance and thus open 

 passages and spaces. The cartilage-cells filling the primary 

 areolse disappear, and communications are made between the 

 areolse by perforations through their calcified walls, though 



