io6 



CHORDATE ANATOMY 



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 bone cell. 



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 



bl.v. i bl.v. 



Fig. 103. — 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., mesen- 

 chymal cell ; X, young fat cell, nucleus not in section; 1,2,3, developing fat cells. (From 

 Bremer, "Text-book of Histology.") 



exceedingly thin layer surrounding the vacuole (Fig. 103). The flat 

 nucleus lies in the peripheral layer of cytoplasm. The irregular poly- 

 hedral form of the cells is doubtless the result of their mutual pressures. 



Blood 



The circulatory function of blood requires that it be fluid but various 

 special services are rendered by cells suspended in the fluid, some of them 

 passively carried by it, others capable of independent motion somewhat 

 like that exhibited by an ameba. 



