LIPID CONTENT AND COMPOSITION OF ANIMAL 579 



livers of the twenty-two-day and thirty-six-day-old chicks varied from 0.3 

 to 0.4%, which approximates the value in mature birds. 



Another variation in the lipid pattern of the chicken immediately after 

 hatching is the fact that the major portion of the cholesterol is in the esteri- 

 fied form. The values of esterified cholesterol were not decreased to that of 

 adult animals until the birds were fourteen days of age. The decrease in 

 cholesterol ester was associated with an increase in the triglyceride content 

 of the liver. 



b. The Effect of Embryonic Development. The lipid composition of the 

 early embryo varies markedly from that of the newborn animal. The 

 phospholipid content has been shown to be highest, on the dry weight basis, 

 in the very young pig fetus 263 ; the proportion of this component drops to 

 50% of the original level at term. The fatty acids in this fraction were 

 shown to have an iodine number of 82. 



Although the non-phospholipid fatty acids have usually been considered 

 to be neutral fat, Gortner 263 reported that they are largely free fatty acids 

 which are present in unesterified form. The fetal glycerides do not begin to 

 assume importance until the middle of gestation, after which they gradu- 

 ally increase until term. However, even in the newborn pig, the glycerides 

 account for only a minor proportion of the total lipids. 



On the other hand, the unsaponifiable lipids in the pig fetus gradually de- 

 crease proportionately as gestation proceeds. There is a parallel drop in 

 the amount of free and total cholesterol. The total lipid and the lipid: 

 protein ratio remain constant for a large part of the embryonic growth 

 period. 



c. The Effect of Sex. (a) The Effect on Total Fat. The fat content 

 of the female tends to be higher than that of the male. Thus, in the ex- 

 periments of Deuel et al., 2i9 the total tissue lipids of female rats on several 

 fat diets exceeded those of males on similar regimens, while the males 

 showed a somewhat higher water and protein content than did the females. 

 Making use of a specific gravity method for the calculation of body fat, 

 Rathbun and Pace 186 found that female guinea pigs average 4.7% more 

 body fat than do the males. Moreover, Jamin and Muller 264 stated that 

 the specific gravity of the partial body volume (not including the head) was 

 higher in men (1.095) than in women (1.081). Presumably, the fat content 

 varies inversely with the specific gravity. 



Although the relationship between sex and lipid content seems to be a 

 general one in different species, there is some evidence that the variation 



263 W. A. Gortner, /. Biol. Chem., 159, 135-143 (1945). 



264 F. Jamin and E. Muller, Munch, med. Wochschr., 50, 1454-1457, 1511-1515 (1903). 



