I 10 THE TISSUES. 



fication becomes hollowed out, and there are formed irregular anas- 

 tomosing spaces, primary marrow spaces, separated by partitions or 

 trabeculae of calcified cartilage matrix. Into these primary mar- 

 row spaces grow the periosteal buds, consisting of small blood- 

 vessels, cells, and some few connective-tissue fibers, forming embry- 

 onic marrow, tissue. Some of the cells which have thus grown into 



teal bone 



ry marrow 



Fig. 86. Longitudinal section of the proximal end of a long bone (sheep embryo) ; 



X3Q. 



the primary marrow spaces arrange themselves in layers on the 

 trabeculae of calcified matrix, which they envelop with a layer of 

 osseous matrix formed by them. The cells thus engaged in the 

 formation of osseous tissue are known as osteoblasts. 



Ossification proceeds from the center of ossification toward the 



