HISTOLOGY OF THE FROG 107 



canals of the Haversian systems. Some of the osteoblasts 

 become imbedded in the advancing bone, and remain as the 

 bone-corpuscles of the future lacunae. 



The so-called ossification in cartilage can be very well 

 studied in a long bone such as the tibia or femur. In the 

 first place, a small model of the future bone is laid down in 

 cartilage, and this is surrounded by a vascular membrane 

 which will become the future covering or periosteum of the 

 bone. When ossification is about to commence the cartilage- 

 cells in the middle of the model become much larger and 

 flatter, and are arranged in a number of rows or columns 

 parallel to the long axis of the bone. The matrix between 

 them becomes calcified by deposition of calcareous granules. 

 In the meantime true bone is being formed outside the 

 cartilage, between it and the periosteum. The last-named 

 membrane is vascular, and contains numerous granular cells, 

 chiefly on its inner surface nearest to the cartilage. From 

 these osteoblasts, osteogenic fibres are formed, and true bone 

 containing blood-vessels and bone-corpuscles is formed just 

 as in the case of membrane bone. At this stage the develop- 

 ing bone consists of a core of calcified cartilage enclosed in 

 its middle portion by a sheath of membrane bone. Presently 

 the vascular and osteoblastic tissues break through the sheath 

 of newly-formed bone and burrow their way into the cartilage, 

 which is for the most part absorbed by the agency of certain 

 giant-cells containing several nuclei, and known as osteoclasts. 

 A network of bony spicules is developed from the intrusive 

 osteoblasts in the cavities formed by the absorption of the 

 cartilage, and this network carries blood-vessels in its meshes 

 just as is the case in membrane bone. Gradually the whole of 

 the cartilage of the shaft is absorbed and replaced by a sponge- 

 work of bone, which for a long time remains spongy or 

 "cancellous," but eventually is solidified in the peripheral 

 parts of the shaft by the deposition of concentric layers round 

 the blood-vessels, whilst the central portion is absorbed, and 

 a central cavity, the medullary space, is left in the middle of 

 the bone. 



Before leaving the subject of connective tissue, mention 

 may be made of fat-cells, which, although their function is 

 essentially metabolic, are nothing more in their origin than 

 connective tissue corpuscles in which droplets of fatty matter 



