122 THE TISSUES. 



in layers on the trabeculae of calcified cartilage, which they envelop 

 with osseous tissue. 



5. Osteoclasts cause the absorption of many of the smaller 

 osseous trabeculae ; others become thickened by a deposition of 

 new layers of osseous tissue. Osteoblasts are enclosed in bone- 

 tissue and become bone-cells. In this way there is formed embry- 

 onic cancellous bone, bounding Haversian spaces inclosing embry- 

 onic marrow. 



6. In the diaphysis, the greater portion of the embryonic can- 

 cellous bone is also absorbed (by osteoclasts) ; the Haversian spaces 

 unite to form a part of the marrow space of the shaft of the bone. 



2. Intramembranous Bone. This, the simpler type of ossifi- 

 cation, occurs in bone developed from a connective-tissue founda- 

 tion, and is exemplified in the formation of the bones of the 



Blood- 

 vessel. 



Ossification 

 ridge. 

 Epiphyseal 

 cartilage. 



Fig. 88. Longitudinal section through epiphysis of arm bone of sheep embryo ; X I; * 

 a, b, Primary marrow spaces and bone lamellae of the diaphysis. 



cranial vault and the greater number of the bones of the face, aid 

 also in bone developed from the periosteum (perichondrium) sir- 

 rounding the cartilage fundaments of endochondral bone. All 

 fibrous-tissue bone is developed in the same way. 



The intramembranous bone-development begins by an apprcxi- 

 mation and more regular arrangement of the osteoblasts of the 

 osteogenetic layer of the periosteum about small fibrous-thsue 

 bundles. The osteoblasts then become engaged in the formation 

 of the osseous tissue which envelops the fibrous-tissue bundles. 

 In this way a spongy bone with large meshes is formed, consisting 

 of irregular osseous trabeculae, surrounding primary marrow spaces. 

 These latter are filled by embryonic marrow and blood-vesses de- 

 veloped from the tissue elements of the periosteum not engaged in 

 the formation of bone. 



