Histology 



327 



Fig. 269. Fat-cells in subcutaneous tissue of a human embryo of four months, 

 (bl.v.) Blood-vessel; (c.t.) white connective-tissue fibers; (fib.) young fibrocyte; 

 (mes.) mesenchymal cell; (x) young fat-cell, nucleus not in section; (1, 2, 3) de- 

 veloping fat-cells. (Courtesy, Bremer: "Text-Book of Histology," Philadelphia, 

 The Blakiston Company.) 



cartilage and bone lies in the fact that the cartilage-cell produces 

 matrix in all directions and thus surrounds itself by its own product, 

 whereas the osteoblast produces matrix only at such part of its surface 

 as is adjacent to the already formed bone. A layer of bone cells build- 

 ing up lamella upon lamella of bone may be likened to a group of 

 masons laying course upon course of stone at the unfinished top of a 

 wall. But, in the case of the bone, every now and then one of the 

 masons, an osteoblast, is left behind and buried between successive 

 courses of the wall, remaining there in his little lacuna as a permanent 

 and living bone-cell. 



Bone once formed may be broken down or "resorbed." By this 

 means, the marrow cavity of a long bone is enlarged as the bone grows. 

 The resorption is attributed to the action of special cells called 

 "osteoclasts." 



ADIPOSE TISSUE 



Adipose tissue or "fat" consists of cells each of which contains a 

 globule or vacuole of oil so large that the cytoplasm appears as merely 

 an exceedingly thin layer surrounding the vacuole (Fig. 269). The flat 



