BONE. 



61 



primary areolae they absorb it, and thus cause a fusion of the original cavities and 

 the formation of larger spaces, which are termed the secondary areolce (Sharpey) 



FIG. 32. Section of fcetal bone of cat. ir. Irruption of 

 the subperiosteal tissue. . Fibrous layer of the perios- 

 teum, o. Layer of osteoblasts. im. Subperiosteal bony 

 deposit. (From Quain's Anatomy, E. A. Schafer.) 



FIG. 33. Part of a longitudinal section of 

 the developing femur of a rabbit, a. Flat- 

 tened cartilage-cells, b. Enlarged cartilage- 

 cells, c. d. Newly-formed bone. e. Osteo- 

 blasts. /. Giant-cells or osteoclasts. g. h. 

 Shrunken cartilage-cells. (From Atlas of His- 

 tology, Klein and Noble Smith.) 



or medullary spaces (Muller). In these secondary spaces the original cartilage- 

 cells disappear, and their cavities become filled with embryonic marrow, consisting 

 of osteoblasts and vessels, and derived, at all events in part, in the manner 

 described above, from the osteogenetic layer of the periosteum (Fig. 33). What 

 becomes of the cartilage-cells is not finally determined. By most histologists they 

 are believed to be converted, after division, into osteoblasts, and so assist in form- 

 ing the embryonic marrow. Others, on the other hand, believe that they are 

 simply absorbed and take no part in the formation of bone. 



Thus far, then, we have got enlarged spaces (secondary areolae), the walls of 

 which are still formed by calcified cartilage-matrix, containing an embryonic 

 marrow, derived from the processes sent in from the osteogenetic layer of the peri- 

 osteum, and consisting of blood-vessels and round cells, osteoblasts (Fig. 33), some 

 of which probably are derived from the division of the original cartilage-cells, which 

 have disappeared. The walls of these secondary areolae are at this time of only 

 inconsiderable thickness, but they now become thickened by the deposition of lay- 

 ers of new bone on their interior. This process takes place in the following 

 manner : Some of the osteoblasts of the embryonic marrow, after undergoing 

 rapid division, arrange themselves as an epithelioid layer on the surface of the 



