g6 Journal of Agricultural Research voi. xvi. No. 3 



relative decreased metabolism of the other solids causes a decrease in 

 the concentration of these solids in the milk. 



By far the best evidence yet presented for the secretion theory for the 

 liberation of the milk solids (the third hypothesis) is given by the diurnal 

 variation of the constituents of cow's milk shown above. It is very 

 difficult to see why the cell constitution should change in balance be- 

 tween the solids-not-fat and butter fat between morning and evening 

 milking on any other theory. Why should the cells discharged in the 

 evening contain the ratio a to 6 of butter fat to solids-not-fat, whereas 

 in the morning the ratio is changed to a markedly lower value of c to b? 

 On the first and second theories the cell must contain a fixed quantity 

 of solids-not-fat, while the butter fat varies in such a way that, in a 

 longer time between the emptying of the gland, the cell accumulates 

 less fat than in the shorter time; or, taken in another way, the cell 

 accumulates relatively more protein as the interval between milkings is 

 lengthened. 



Our knowledge of fat formation by other cells of the body makes it 

 probable that either of these two possible alternatives for the formation 

 of this milk fat on the cell-destruction hypothesis are inconsistent with 

 the facts. In the formation of fat, the cell is first composed chiefly of 

 protein material. In this protein material the fat is accumulated in 

 ever-increasing amounts at the expense and crowding out of the protein 

 constituents. This change of the ratio of the fat to solids-not-fat is 

 just opposite to what must take place in the mammary gland on the 

 cell-destruction hypothesis. The ratio of the fat to the solids-not-fat 

 in the fat cells increases as the cells increase in age ; the ratio of the fat 

 to the solids-not-fat in milk, derived from the cell breakdown on the 

 two destruction hypothesis, decreases as the age of the cell increases. 

 It is hard to believe that there is such a difference in fat formation going 

 on in the body. It is much more likely that the mechanism of fat forma- 

 tion in the two cases is the same and that the diurnal variation of the 

 ratio of butter fat to solids-not-fat is only another phase of the changes 

 which take place in known secretions. The large variations current in 

 the amount of milk produced, the variations of the constituents with 

 age, the great and characteristic differences in the composition of the 

 milk of two cows of the same breed all add weight to the view that milk 

 is secreted. By analogy, the mammary-gland mechanism for milk 

 secretion would agree with practically all of the glands which secrete. 

 There is no need to assume that rapid cell division is taking place in order 

 to maintain the necessary number of cells for breakdown, as called for 

 on the cell-destruction hypotheses, where no such amount of mitosis as 

 would be necessary is witnessed microscopically. Further, the secretory 

 theory has supporting evidence of whole granules being secreted into 

 the saliva by the salivary glands in much the same way as is done in the 

 secretion of butter fat. The data supporting the secretory hypothesis 



