74 



HISTOLOGY 



uoies. (After LEWIS.) 



they are found among practically all of the mesodermal parts of the 



body, acting also in some degree as buffers and fillers for otherwise 



unoccupied spaces. Their content is often 

 called upon for food when anything goes 

 wrong with the other means of nourish- 

 ment, and they give up this substance 

 readily to the blood to be carried where 

 it is needed. 



The fat of the mammal may be best 

 studied by tracing its genesis in the em- 

 bryo (Fig. 75). We shall do this in the 

 human embryo, beginning with sections of 

 the skin from a foetus of between four and 

 five months' development. These cells are 

 at first exactly the same as the other con- 

 nective-tissue cells lying around them. 

 When they begin to differentiate, a num- 



ber of tiny droplets of the fatty substance appear in their bodies, 



and as these droplets grow in size they push the nucleus to one side. 



In growing, they merge together 



until they form but one large drop. 



For some time after the large drops 



of fat are formed, other small and 



new drops arise in the cytoplasm, 



and later join the large single drop, 



until, at the maturity of the drop, it 



is many times the size of the origi- 



nal cell and shows the cytoplasm of 



this cell lying around it as a thin 



cover, thickened enough at one side 



to contain the nucleus. Enough 



of the undifferentiated connective- 



tissue cells remain in the tissue 



to hold it more or less firmly 



together. Figure 76 is drawn from 



the integumental fat of the chicken 



in which these relations are par- 



ticularly favorable for observa- 



tion. 

 ^ There are many cells in the in- 



vertebrate forms that store up pre- 



pared food materials. In no case, however, is this material exactly like 



the vertebrate fat in chemical composition. In some cases it more nearly 



FIG. 76. Fat cells lying in connective-tissue 

 reticulum. Skin of chicken, nu., fat cell 

 nuclei. X 870. 



