n8 



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 



Groove of 

 ossification. 



Periosteum. 



Periosteal bone 



lamella. 



Primary marrow 

 spaces. 



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



X3- 



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 



