v INTERNAL EESTITUTIVE SECRETIONS 325 



surrounding the fat, which is a little denser at the point where 

 the displaced nucleus lies. After the fat has been removed by a 

 proper solvent, the fat-cell appears as a connective tissue corpuscle 

 of which the cytoplasm is converted into a large vacuole. 



The analogy between fat-cell and gland-cell, e.g. hepatic cells, 

 is obvious. As the one stores up sugar in the form of glycogen, so 

 the other collects fat. The sole difference is that in the liver-cell 

 the glycogen is arranged as a hyaline mass between the granules 

 and the network of cytoplasm ; in the fat-cells, on the contrary, 

 the fat flows together in a single mass, forming a large vacuole in 

 the midst of the cytoplasm and pushing the nucleus of the cell to 

 the periphery. 



The analogy between the liver -cells and fat -cells is more 

 striking when we reflect that both have, respectively, the power, 

 not only of storing up the carbohydrates, or the fats, of alimentary 

 origin, but also of forming these substances by their specific 

 metabolic activity from other materials. 



The ordinary plan of fattening stock-animals, e.g. pigs and 

 geese, with an excess of carbohydrate food, in which there is 

 little fat, shows plainly enough that not the whole of the fat 

 accumulated in the body, but only a small fraction of it, is 

 derived from the fat given in the diet. Moreover, we must take 

 into account that the fat accumulated in different animals differs 

 somewhat in its composition. It consists mainly of olein, palrnitin, 

 and stearin, in variable proportions, with small amounts of the 

 glycerides of butyric, capronic, caprylic, and other fatty acids, 

 united with a little phosphorated substance (lecithin and jecorin) 

 and also cholesterol, which, while it has some of the properties of 

 the fats, belongs by its constitution to the alcohol group. 



Owing to the different proportion in which olein, palmitin, and 

 stearin are present, the body fat of different animals is distinguished 

 by different melting-points. Thus the subcutaneous fat of man 

 melts at 15-20 C., that which surrounds the kidneys only at 25 ; 

 the fat of dogs at 22 ; of ducks at about 28 ; of the ox at about 

 40 ; of the sheep at 50 : olein, which melts most readily, pre- 

 dominates in the first ; stearin, which is least fusible, in the last. 

 This different constitution of the fats depends, not on differences 

 of diet, but on the differences of metabolic activity in the living 

 cells by which it is formed. The diet can in fact be considerably 

 varied, without perceptibly affecting the composition of the fat re- 

 serves in different animals. Owing to this fact, i.e. that the fat of 

 each species of animal has a definite melting-point, it was long held 

 that alimentary fat did not give rise directly to the fat of the body. 

 This, however, was proved by subsequent researches to be erroneous. 

 Kiihne suggested as a decisive experiment in regard to the 

 origin of tissue fat from alimentary fat, that it might be possible 

 to get some fat extraneous to the body stored up by alimentation. 



