CHAP, iv.] METABOLIC PEOCESSES OF THE BODY. 605 



fat-cells. In the embryo, in a situation where adipose tissue is 

 about to be formed, the connective tissue is seen to contain a 

 number of small nucleated cells, rounded or somewhat irregular 

 in form, the cell-substance of which at first presents no special 

 characters, and contains not more than what may be called the 

 ordinary amount of fat globules or spherules. Very soon how- 

 ever these minute drops or specks increase in number, the cell- 

 substance at the same time increasing in bulk while remaining 

 round or becoming more distinctly so, and the smaller drops run 

 together into larger ones. This goes on ; the fat increasing in 

 quantity coalesces more and more, and the cell, as a whole, 

 becomes larger and larger, the cell-substance at first keeping 

 up in bulk with the increasing fat, but subsequently ceasing to 

 increase, being apparently used up in the formation of the fat. 

 Thus the original small ' protoplasmic ' cell is at last transformed 

 into the larger fat-cell, all the fat having run together into a 

 vesicle the envelope of which, thickened on one side to carry 

 the nucleus, is furnished by the remnant of the cell-substance. 

 In some cases, the nucleus instead of being pushed early on one 

 side, remains central though the collection of fat has become 

 considerable ; it is however eventually displaced. The whole 

 process appears very similar to the deposition of mucin in the 

 cells of a mucous gland, 197 ; and we may by analogy infer 

 that the fat-cell becomes a fat-cell by the cell manufacturing fat 

 in some way or other, and depositing the fat so formed in the 

 interstices of its substance. The most striking superficial dis- 

 tinctions seem to be that in the mucous cell the granules or 

 spherules remain discrete within the cell, being separated by 

 bars of cell-substance, whereas in the fat-cell the globules, as they 

 form, run together until at last they unite into a single mass; and 

 further that while in the mucous cell, even when most heavily 

 loaded, a relatively large amount of active cell-substance still 

 remains, in the fat-cell a mere remnant is left and that chiefly 

 surrounding the displaced nucleus. 



The fat in the interior of bones forming the yellow marrow 

 appears to have the same general structure and to be formed in 

 the same way as the rest of the adipose tissue. 



398. The fat thus deposited in a fat-cell sooner or later 

 disappears. It is not ejected bodily into the surrounding 

 lymph-spaces of the connective tissue, but passes away grad- 

 ually either into the lymphatics or into the blood stream by 

 some processes not as yet fully understood. During the disap- 

 pearance of the fat the cell behaves in one of two different 

 ways. On the one hand, as the fat gradually disappears, little 

 by little, the rounded distended vesicle gradually lessening 

 assumes the characters of a connective tissue corpuscle, even 

 of a branched one. On the other hand, especially when the 

 disappearance is rapid and total, the space previously occupied 



