FAT 73 



core of connective tissue containing a blood supply. This core then 

 operates with its peripheral osteoblasts to form an Harversian system, 

 just as the osteoblasts did in the cartilage channels during endochondral 

 bone formation. The rising edges of the future Harversian systems 

 are laid down in a connective-tissue basis that precedes them. This 

 is shown in Figure 74. 



One other histological process in connection with bone formation 

 should be studied here. As most bones increase in diameter, they become 

 hollow in the center, this space being filled with the marrow. In order 

 that this may occur, some of the bone material must be removed, and this 

 occurs as follows. 



Large, heavy cells appear in the Harversian canals and, applying 

 their bodies to the bone material, they proceed to absorb it and remove 

 it. They probably do this by secreting some acid in the cytoplasm and 

 dissolving the salts, which are then carried away by the blood. It would 

 be interesting to know more about the acids that are used and what 

 becomes of the salts in solution; they may be used over again by the 

 osteoblasts. These large cells that remove the bone substance are 

 called the giant bone cells or-osteoclasts (see Fig. 72, gi.c.). They 

 usually contain several nuclei. 



Technic. The technic of this group of tissues is. somewhat diffi- 

 cult on account of the hardness of some of them. To see bone properly, 

 one should study both decalcified and ground sections of the tissue. 

 Developing bone is easily cut in both paraffin and in celloidin when the 

 fixing fluid has been an acid one like Flemming's fluid and most of the 

 others. Any of these tissues may be first fixed and subsequently decal- 

 cified with nitric acid in connection with phloroglucin (see Lee). Some 

 of the shells must be ground, and then the soft parts are nearly always 

 destroyed. Many of the tougher integuments may be cut, at some 

 expense in the sharpening of knives. 



LITERATURE 



Read the general literature on the simple rigid forms and then see the descriptions of 

 bone and its histogenesis in some good medical histology. 



FAT 



Various kinds of cells can store up prepared food materials in their 

 cytoplasm as a reserve supply in the economy of the animal. Chief 

 among these are the otherwise unspecialized connective-tissue cells of 

 the vertebrates that are able to take such materials into the cytoplasm 

 in the form of various fatty acids. These are known as the fat cells, and 



