ADIPOSE TISSUE 73 



When examined fresh, each fat cell appears as a large round oil-drop, 

 which is more or less compressed into a polyhedral shape by the sur- 

 rounding cells. It is highly refractive, having a border which becomes 

 alternately bright and dark on changing the focus. The liquid fat or 

 oil which fills the cell, leaving only an imperceptible film of protoplasm 

 around it, may escape by the rupture of the membrane, thus forming 

 smaller drops. In the specimen shown in Fig. 52 the fat was seen coming 

 out from the upper surface of one of the cells, and the droplets thus 

 emerging ran together forming larger ones. As fat cells develop, 

 a coalescence of small drops occurs in the protoplasm. 



The earliest formation of adipose tissue is said to occur in human 

 embryos of the fourth month. It may be studied advantageously in 

 the subcutaneous tissue of embryos of the fifth month (Fig. 58). In 

 such specimens there are areas of loose and very vascular mesenchyma, 

 found at the level of the roots of the hairs, in which certain cells exhibit 

 vacuoles. These cells are at first quite like the surrounding fibroblasts, 



FIG. 58. SUBCUTANEOUS FAT CELLS FROM A HUMAN FIG. S9. FAT CELLS FROM NEAR THE KIDNEY OF 



EMBRYO OF THE FIFTH MONTH. X 520. A NEW-BORN CHILD. X 520. 



n Nucleus; f.v., fat vacuole; p. r., protoplasmic rim. 



being fusiform or stellate. Their protoplasm contains several small 

 vacuoles, some of which unite to form one large drop, and the nucleus 

 together with the greater part of the protoplasm, is pushed to one side 

 (Fig. 58, n). Sections of such cells have the form of "signet rings." 

 Frequently small vacuoles are seen in the accumulation of protoplasm 

 beside the nucleus. With further development the fat droplet becomes 

 so large that the protoplasmic rim appears as a mere line or membrane, 

 just within which is the greatly flattened nucleus. During the formation 

 of the fat cells, the branching processes become very short, but it is 

 doubtful whether they are altogether lost. 



For some years after birth fat cells containing several vacuoles are 

 found in certain situations, as around the kidney (Fig. 59) and in the 

 outer layer of the oesophagus. Usually these are regarded as immature 

 forms. 



Adipose tissue of the adult, when well preserved, presents cells of 



