The precursors in cows’ blood of milk fat and other milk constituents

There can be no doubt that some constituent or constituents of the circulating blood undergo synthesis or transformation in the mammary gland into milk fat, with its characteristic chemical and physical properties. Possible origins of the milk fat may be arranged in the following a priori order of probability. Milk fat may be formed (1) by transformation of blood fat, i. e., the circulating triglycerides; (2) by resynthesis from blood phospho-lipin (lecithin, kephalin, or sphingomyelin (3) from blood sugar (providing the glycerol) and cholesterol esters (providing the fatty acid) with the liberation of free cholesterol in the venous blood; (4) from blood sugar (providing the whole of the milk fat by extensive chemical transformations); (5) by some combination of two or more of the foregoing; (6) by some hitherto undescribed transformation.