810 VII. LIPID DISTRIBUTION IN SPECIFIC TISSUES 



Moreover, the glycerol effect has been shown to occur without an increased 

 glucose uptake. 



There is considerable evidence that glycerol can originate from glucose 

 in the udder. French and Popjak 558 demonstrated that the glycerol in the 

 milk of lactating rabbits obtained six hours after C 14 -glucose had been given 

 contained a large proportion of the carbon isotope. Although a similar 

 condition existed after labeled acetate had been given, the level of specific 

 activity was much higher when glucose was used. The incorporation of 

 C 14 into lactose follows the administration of acetate containing isotopic 

 carbon. However, Folley 518 is of the opinion that the glycerol synthesized 

 from acetate arises from glucose rather than from lactose as the interme- 

 diate. Although the rat and sheep were able to produce radioactive glycerol 

 from labeled acetate when the tests were carried out on the intact animal, 

 Balmain and co-workers 572 were unable to duplicate the results in in vitro 

 tests with slices of mammary tissue. 



15. Lipids Present in Eggs 



While the ova of mammals as well as the eggs of fishes contain a large 

 proportion of nucleic acid and only small amounts of lipids, those of the 

 fowl and of reptiles contain relatively large amounts of this foodstuff, in 

 addition to the egg cell. During the course of multiplication of the single 

 egg cell, with the corresponding development of the embryo, the compo- 

 nents stored in the yolk and white of the egg furnish basic materials for the 

 development of the cells, and at the same time provide the essential energy 

 for such growth. Whereas milk provides the nutrients essential for the 

 growth and development of the young animal which is -already completely 

 formed at birth, the egg supplies the material essential for the embryo from 

 the beginning of development. The chief variation in lipid composition 

 between milk and eggs is that the latter supply a large proportion of 

 cholesterol and phospholipids, while these components are largely absent in 

 milk. Apparently, the embryo must be furnished with these requisites at 

 an early stage in the process of development, until it is able to synthesize 

 them. In the case of the mammal, these essential lipids are supplied to the 

 fetus in the mother's blood 587 ; since this is not possible in the case of the 

 developing chick, the storehouse of reserve materials available in the egg 

 yolk must contain an adequate supply. 



687 E. M. Boyd and K. M. Wilson, J. Clin. Invest., 14, 7-15 (1935). 



