172 



TEXT-BOOK OF EMBRYOLOGY. 



vessels, and first appears in the axilla and groin about the thirteenth week. 

 It is formed in other places at later periods, even during adult life, but the 

 mode of development is always the same. In some of the cells in the neigh- 

 borhood of small blood vessels minute droplets of fat are deposited. The 

 origin of the fat is not known. The droplets become larger, other smaller ones 

 appear, and finally all of them coalesce to form a single large drop which practi- 

 cally fills the cell. The result of this is that the remaining cytoplasm is pushed 

 outward and forms a sort of pellicle around the fat. The nucleus also is 

 crowded outward and comes to lie flattened in the pellicle of cytoplasm (Fig. 



Small artery 



Fig. 148. — Developing fat from subcutaneous tissue of pig embryo 5 inches long. Small 

 artery breaking up into capillary network' groups of fat eel's developing in 

 embryonic connective tissue 



149) . At the same time the whole fat cell increases in size and forms a relatively 

 large structure. 



Fat cells usually develop in groups or masses around blood vessels (Fig. 

 148). The neighboring groups gradually enlarge and approach each other, 

 but do not fuse, thus leaving more or less fibrous connective tissue between 

 them, which constitutes the interlobular tissue seen in adult adipose tissue. 

 Among the individual cells in a lobule there is also a small amount of fibrous 

 tissue present. From the mode of development a small artery usually affords 

 the blood supply for each lobule. 



Cartilage. — In the different kinds of cartilage the matrix probably repre- 



