92 



THE TISSUES 



character of the tissue. The adult fat cell is surrounded by a distinct 

 cell membrane, and almost the entire cell is occupied by a single 

 droplet of fat (Figs. 37 and 38). The nucleus, flattened and sur- 

 rounded by a small amount of cytoplasm, is usually found pressed 

 against the cell wall (Fig. 38). This appearance of a distinct cell 

 membrane enclosing the spherical fat droplet, with the nucleus and 

 cytoplasm pressed into a cresent-shaped mass at one side, has given 

 rise to the term "signet-ring cell." Fat cells which occur singly, 



Fig. 36.- — Fat Tissue from Human Subcutaneous Tissue (Child) to show Lobulation. 



X25. (Technic i, p. 99.) 



or in small groups, or in the developing fat of young animals are 

 spherical (Fig. 37). In large masses of adult fat, the closely packed 

 cells are subjected to pressure and are polyhedral (Fig. 38). Fat 

 cells are usually arranged in groups or lobules, each lobule being 

 separated from its neighbors by fibrillar connective tissue (Fig. 36). 

 The appearance which adult fat presents can be understood only 

 by reference to its histogenesis. Fat cells are developed directly 

 from embryonic connective-tissue cells. In the human embryo 

 they are first distinguishable as fat cells about the thirteenth week. 

 The connective-tissue cells which are to become fat cells gather in 

 groups in the meshes of the capillary network which marks the ending 

 of a small artery. Each group is destined to become an adult fat lobule 

 (Fig. 39). 



