BONE. 



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free the cartilage cells as the lacunae are broken down. This has occurred 

 in Fig. 66. The tissue which enters the cartilage is a vascular, embryonic 

 connective tissue, containing osteoblasts, and forming the primary marrow. 

 Meanwhile the cartilage has continued to grow, especially in length, and 

 the cells in the calcined region have divided so as to form rows. The 

 transverse walls of the lacunae are dissolved, leaving deep blue spicules 

 of calcified matrix extending from the ends of the cartilage toward its 

 center. Osteoblasts arrange themselves on these spicules and form bone, 

 the matrix of which stains red with eosin. It was formerly thought that 



Haversian Endochondral Perichondral 



Periosteum. depressions. bone. 'bone. 



Haversian canal. 



Calcified matrix 

 between endo- 

 chondral and 

 perichondral 

 bone. 



Blood vessel. 



Marrow . 



Remains of calci- " 

 fied matrix of 

 cartilage. 



FIG. 68. FROM A CROSS SECTION OP THE SHAFT OF THE HUMERUS, FROM A FOUR MONTHS' 

 HUMAN FETUS. X 80. 



the cartilage cells set free by the absorption of the walls of the lacunae 

 became osteoblasts, but now they are considered as dying cells without 

 further function. The osteoblasts belong with the invading cells. As 

 seen in Fig. 67, both the perichondral bone on the surface of the cartilage 

 and the endochondral bone forming within it, develop like membrane bone. 

 As the bone grows, the older parts which have formed around the calcified 

 cartilage are resorbed, and in the shafts of adult bones probably 

 no trace Df the cartilage remains. In the ear bones, however, calcified 



