THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. . 177 



the original embryonic connective tissue On the surface of the cartilage a 

 membrane of dense fibrous connective tissue, known as the perichondrium, 

 develops (Fig. 153). In most cases, ossification begins about the middle 

 of the piece of cartilage, corresponding to the middle of the shaft of a long 

 bone (Fig. 153). The cell spaces enlarge and in some cases the septa of matrix 

 between the enlarged spaces break down, so that several cells may lie in one 

 space. The cell spaces radiate from a common center, but a little later they 

 come to lie in rows parallel with the long axis of the mass of cartilage. During 

 these early changes lime salts are deposited in the matrix of the cartilage in 

 this region, and the portion so involved is known as a calcification center. 



So far the process is preparatory to actual bone formation. Then small 

 blood vessels from the perichondrium (periosteum) grow into the cartilage, 



Periosteal bud Blood vessel 



Cartilage 



cell spaces ^~~ ^ Cartilage cells 



^^ * 



)*rTr 7 ' 



~\^ Cartilage matrix 



ep,,. .$$ -*'jv 



Periosteum 



FIG. 154. From section of one of the tarsal bones of a pig embryo Showing periosteal bud 

 pushing into the cartilage at the ossification center. (Intracartilaginous ossification.) 



carrying with them some of the embryonic connective tissue. These little 

 ingrowths of connective tissue and blood vessels are known as periosteal buds 

 (Fig. 154). The septa between the enlarged cartilage cell spaces break down 

 still further, forming still larger spaces into which the periosteal buds grow. 

 Many of the connective tissue cells are transformed into osteoblasts oval or 

 round cells with distinct nuclei and a considerable amount of cytoplasm and 

 with the fibers and blood vessels constitute osteogenetic tissue (Fig. 155). The 

 cartilage cells in this region disintegrate and disappear, and the cavity formed 

 by the coalescence of the cell spaces constitutes the primary marrow cavity (Fig. 

 155). From the primary marrow cavity osteogenetic tissue pushes in both 

 directions toward the ends of the cartilage. The transverse septa between the 

 enlarged cartilage cell spaces break down, leaving a few longitudinal septa 

 which form the walls of long anastomosing channels which are continuous With 



