182 BONE AND BONE MABROW 



only line the primitive periosteum but also form budding processes 

 which project into the adjacent connective tissue. 



Bone tissue is now formed by the osteoUasts. The deposit of 

 the fibrous stroma precedes calcification and in this way brushes 

 of radiating fibres frequently project from the osteogenic buds 





c 



FIG. 169. INTRAMEMBRANOUS BONE FORMATION IN THE LOWER JAW OF AN EMBRYO 



SHEEP. 



a, bone ; &, primary marrow cavity ; c, osteoblasts ; eZ, growing point of the primitive 

 bone, beyond which primary marrow is developing in the connective tissue of the meso- 

 blast. x 300. (After Bohm and von Davidoff.) 



beyond the limit of the calcareous deposit. The bony processes 

 thus formed are covered by a layer of osteoblasts which continue 

 to deposit bony lamellae, with their bone corpuscles and lacunae, 

 in the same manner as in enchondral bone formation : a mass of 

 spongy bone results. 



The ma/rroiv spaces of the cancellous bone are occupied by 

 embryonic connective tissue in which are many small and thin- 

 walled blood vessels. This primitive marrow differs from that of 



